Super stainless steel
At the end of the twentieth century, both domestic and foreign countries began to develop stainless steel varieties, and the existing four major stainless steel classes have trial-produced “super” grades, the common characteristics of which are: to ensure a more stable organization from the composition design; Excellent corrosion resistance in more demanding environments; Better mechanical properties. Molybdenum not only meets the above requirements, but also is an important alloying element to replace expensive high-composite materials, and most of the four types of “super” grades of stainless steel are added with the alloying element molybdenum.
Chemical composition of “Super” grade stainless steel,%
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Stainless steel
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Cr
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Mo
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N
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Nor
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Mn
|
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Ultra-biphasic
SAF2507
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25
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4
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0.3
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7
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1.2
|
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Super austenite
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25
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5
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0.2
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25
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|
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Superferrite
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29
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4
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–
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–
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|
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Hypermartensite (Alloy 1)
X80 11Cr-2Ni
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11
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< 0.5
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< 0.012
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2.0
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< 2.0
|
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Hypermartensite (Alloy 2)
X80 12Cr-4.5Ni-1.5Mo
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12
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1.5
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< 0.012
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4.5
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< 2.0
|
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Hypermartensite (Alloy 3)
X80 11Cr-6.5Ni-2.5Mo
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12
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2.5
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< 0.012
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6.5
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< 2.0
|
Super duplex stainless steel type, containing high molybdenum and nitrogen, standard grade UNSS32750 (25Cr-7Ni-3.7Mo-0.3N), some also contain tungsten and copper, PREN value greater than 40, can be applied to harsh media conditions, with good corrosion resistance and mechanical comprehensive properties, comparable to super austenitic stainless steel.
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Super DSS
Super
duplex
stainless
steel
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US S32760
Wr.Nr1.4501
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Zeron100
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0.03
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25
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7
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3.5
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0.24
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0.7
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0.7
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40
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41.5
|
|
|
|
|
|
|
|
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|
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||
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US S32750
Wr.Nr1.4410
SS 2328
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SAF2507
UR47N+
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0.03
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25
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7
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3.8
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0.28
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–
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–
|
41
|
|
|
|
|
|
|
|
|
|
|
|
|
|||
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|
|
|
|
|
|
|
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|||
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US S32550
Wr.Nr1.4507
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UR52N+
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0.03
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25
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7
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3.5
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0.25
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1.5
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–
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41
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|
|
|
|
|
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|
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||
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US S32740
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DP3W
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0.03
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25
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7
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3
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0.27
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–
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2.0
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39
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42.5
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1. Typical chemical composition of supermartensitic stainless steel
In order to adapt to the different conditions of use of this new supermartensitic stainless steel, the Belgian company FaEER has researched and designed three alloys with different chemical compositions
In recent years, stainless steel production plants in various countries have made great efforts in the development of low-carbon, low-nitrogen supermartensitic stainless steel, and have produced supermartensitic stainless steel suitable for different uses.
Target chemical composition of the three alloys/%
|
Element
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Alloy 1
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Alloy 2
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Alloy 3
|
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C
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< 0.015
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< 0.015
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< 0.015
|
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Mn
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< 2.0
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< 2.0
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< 2.0
|
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P
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< 0.030
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< 0.030
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< 0.030
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S
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< 0.020
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< 0.002
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< 0.002
|
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Yes
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0.15
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0.15
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0.15
|
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With
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0.40
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0.40
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0.40
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Nor
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2.0
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4.5
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6.5
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Cr
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11.0
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12.0
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12.0
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Mo
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< 0.5
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1.5
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2.5
|
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N
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< 0.012
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< 0.012
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< 0.012
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2. Microstructure of superhorse stainless steel
The typical base metal microstructure of ultramartensitic stainless steel is tempered martensite, which has high strength and toughness. According to the difference of different heat treatment conditions with nickel content, 10%~40% fine diffuse residual austenite may appear in the microstructure of some grades of ultramartensitic stainless steel. For ultramartensitic stainless steels with a chromium 16% grade, small amounts of δ ferrite may appear in the microstructure. In order to obtain the ideal fine-grain tempered martensite, it is generally quenched and tempered before the steel plate is delivered.
Typical chemical composition of supermartensitic stainless steel/%
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trademark
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Chemical components
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Production plant
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||||||||
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C
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Mn
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Yes
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Cr
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Nor
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Mo
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With
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N
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other
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||
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HP13Cr
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＜0.03
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0.4
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＜0.3
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13
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4
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1
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–
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0.05
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–
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Kawasaki
|
|
Super 13Cr
(13-5-2)
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0.02
|
0.4
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0.2
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12.5
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5
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2
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–
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＜0.08
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–
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Sumitomo Metals
|
|
Super 13Cr
(13-6-2.5-Tue)
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＜0.01
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0.4
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0.3
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12
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6.2
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2.5
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–
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＜0.01
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Ti
0.07
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Sumitomo Metals
|
|
13Cr
(12-5-2)
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0.02
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0.5
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0.2
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12.2
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5.5
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2
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0.2
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0.02
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V0.2
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British Steel Union
|
|
D13.5.2N
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0.02
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0.7
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0.3
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13.3
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4.8
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1.6
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0.1
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0.08
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–
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Darming
|
|
CRS(＞95ksi)
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0.02
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0.5
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0.3
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12.5
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4.5
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1.5
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1.5
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0.05
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–
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Nippon Steel
|
|
CRS(＞110ksi)
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0.02
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0.5
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0.3
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12.8
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5.9
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2
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1.5
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0.02
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–
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Nippon Steel
|
|
X80 11Cr-2Ni
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＜0.015
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＜2
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0.15
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11
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2
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＜0.5
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0.4
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＜0.012
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–
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Charleroy
|
|
X80 12Cr-
4.5Ni-1.5Mo
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＜0.015
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＜2
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0.15
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12
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4.5
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1.5
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0.4
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＜0.012
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–
|
Charleroy
|
|
X80 12Cr-
6.5Ni-2.5Mo
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＜0.015
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＜2
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0.15
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12
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6.5
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2.5
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0.4
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＜0.012
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–
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Charleroy
|
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248SV
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0.03
|
|
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16
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5
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1
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|
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Fe
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Pander to. Sheffield
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3. Mechanical properties of superstructure stainless steel
Super martensitic stainless steel not only has good corrosion resistance and weldability, but also has the characteristics of high strength and good low-temperature toughness. Typical mechanical properties are as follows:
The yield stress б0.2 is 550~850MPa
The tensile strength бb is 780~1000MPa
Impact strength greater than 50 joules
Elongation greater than 12%
Avista. Physical and mechanical properties of 248SV martensitic stainless steel produced by Sheffield
Physical and mechanical properties of 248SV
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Density/kg·m-20 at 3°C
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7700
|
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Thermal conductivity/W· (m·K) -1
|
22
|
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比热J/· (kg· K)-1
|
460
|
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Coefficient of linear expansion at 20-100°C/10-6· K-1
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11
|
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Resistivity/mΩ·m
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0.6
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Modulus of elasticity/GPA
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215
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* Low yield strength б0.2/Mpa
|
|
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20℃
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620
|
|
100℃
|
610
|
|
200℃
|
590
|
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300℃
|
570
|
|
400℃
|
540
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* Low yield strength (20°C)б0.1/MPa
|
660
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* Low tensile strength (20°C)/Mpa
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830
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*Low elongation A5/%
|
15
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*Low impact strength/J
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59
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Hardness/HB
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260-320
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In order to obtain good low-temperature performance, it is extremely important to reduce the ferrite content in microtissues, or even eliminate it completely. To this end, it is necessary to control all processing processes well to avoid affecting the final chemical composition of the product during these processing, which is not conducive to the control of steel microstructure.
Alloy 1 listed in Table 1 can meet the requirements of -20C impact performance, and alloy 2 and alloy 3 can meet the requirements of -40C impact performance. In order to fully demonstrate the toughness of supermartensitic stainless steel, the Belgian Welding Institute (located in Ghent) has completed a large number of tests on different parts made of 13Cr, especially the welds on it.
Can weld 13Cr stainless steel toughness test results
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Test method
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Test materials
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Test temperature
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Test results
|
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Summer’s V-notch
Impact test
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Midplate
|
-20℃
|
150-200J
|
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Welds
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100-130J
|
||
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Heat affected zone
|
45-100J
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||
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Wide specimen
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Welds
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-20℃
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1.0mm
|
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Heat affected zone
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0.5mm
|
||
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Crack tip gap
Displacement test
（CTOD）
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Midplate
|
-20℃
|
|
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Welds
|
0.15-0.25mm
|
||
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Heat affected zone
|
0.15-0.25mm
|
Through the rigorous demonstration of different test methods, it has been proved that as long as the sample does not have various artificial gaps, various methods have obtained good test results. This fully proves that the low-temperature toughness of weldable 13Cr stainless steel is very good.
When supermartensitic stainless steel is forged, there are no special requirements for forging temperature and forging force. Even if the forging temperature is low, crack-free forging can be obtained. This clearly shows that supermartensitic stainless steels have good malleability. 4. Welding performance of super martensitic stainless steel
Martensitic stainless steel contains 13% chromium, making it very weldable due to the coarsening and hardening of the ferritic grains during welding. In the past, the use of this type of steel was strictly limited to the installation of non-welded parts. After the continuous improvement of the smelting process, the carbon, nitrogen and sulfur content in martensitic stainless steel was reduced, and a certain amount of nickel (* high content up to 6.5%) and molybdenum (* high content 2.5%) was added, so as to develop super martensitic stainless steel. This material has taken special technological measures in the process of processing and manufacturing, so that the welding performance of the new supermartensitic stainless steel greatly exceeds that of the traditional martensitic stainless steel.
The welding of supermartensitic stainless steels can use familiar welding processes, such as gas shielded metal arc welding (GMAW or SMAW), gas shielded tungsten arc welding (GTAW). Submerged arc welding (SAW) and field coil arc welding (FAW). GTSW, GMAW and SAW can be used for circumferential welding, and most straight seam welding uses SAW. However, laser welding and electron beam welding are also very attractive. Laser welding pairs to produce straight seam welded pipes is an economical welding method. Due to the fast cooling rate, the whole martensitic microstructure can be obtained in the weld, so as to obtain good toughness and satisfactory corrosion resistance.
In order to ensure that the weld has high toughness and good corrosion resistance, the welding wire currently used is mostly duplex stainless steel and super duplex stainless steel. However, because it is different from the base material, the weld has uneven corrosion, and duplex stainless steel or super duplex stainless steel is much more expensive than super 13Cr stainless steel, so people are working to develop welding wires that match super martensitic stainless steel to replace duplex stainless steel or super duplex stainless steel welding wire.
Low carbon and low nitrogen supermartensitic stainless steels can be used in welded conditions. Some martensitic stainless steel grades (especially those with additional nitrogen) should be heat treated after weld in order to obtain lower hardness and better toughness.
The effect of post-weld heat treatment depends on the chemical composition of the base metal, the heat treatment time, the temperature, and the wire material used (whether it is duplex stainless steel or a material that matches the base metal). When formulating the heat treatment process and hot processing heating temperature, we should pay great attention to the * high heating temperature, if the * high heating temperature is not properly controlled, it is easy to have grain growth, resulting in a significant decrease in toughness. Of course, in-depth research on this type of steel has found that even if very coarse grains appear, they can be re-quenched and tempered to refine the grains. For example, refine from ASTM 2 to ASTM 5 or 6. This is also one of the advantages of Super Cr13 stainless steel.
5. Corrosion resistance of super martensitic stainless steel
Super martensitic stainless steel has good corrosion resistance due to its low carbon content, which is equivalent to increasing the proportion of chromium content in the base metal.
For weakly acidic corrosion environments, supermartensitic stainless steels have a tendency to replace other corrosion-resistant alloys. However, under the corrosion conditions of high temperature and the presence of carbon dioxide, general corrosion and local corrosion will occur, and under the corrosion conditions of carbon dioxide and hydrogen sulfide at the same time, stress corrosion cracks generated at room temperature and local corrosion and general corrosion generated at high temperature must be considered. Depending on the corrosion conditions (e.g.CO2 or CO2+H2S), different grades of supermartensitic stainless steels (molybdenum-containing or molybdenum-free) have been developed. Alloy 1 in Table 1 of this article is suitable for environmental conditions that do not contain acid and do not have hydrogen sulfide; Alloy 2 is suitable for environmental conditions with weak acids,* large hydrogen sulfide content of 10ppm; Alloy 3 is suitable for environmental conditions with weak acids,* large hydrogen sulfide content of 50ppm. This ultra-13Cr stainless steel has been successfully applied in the environmental conditions of the Gullfaks field and the Asgard field in the North Sea region. Avista. 248SV Markov developed by Sheffield Company
The resistance of general corrosion and pitting corrosion of stainless steel is better than that of 13% chromium and 17% chromium martensitic stainless steel, and is comparable to 304 type Olympic stainless steel, which indicates that supermartensitic stainless steel has better corrosion resistance.
Environmental conditions in oil fields
|
|
Gullfaks field
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Asgard field
|
|
Design pressure/bar
|
390
|
390-475
|
|
Operating pressure/bar
|
200-250
|
200-475
|
|
Operating temperature/°C
|
90
|
60-140
|
|
CO2 concentration/%
|
< 2
|
< 6
|
|
Hydrogen sulfide content/ppm
|
4-10
|
10-40
|
Super martensitic stainless steel is also known as soft martensitic stainless steel, and some are also called weldable martensitic stainless steel or 13Cr stainless steel.
Traditional martensitic stainless steel usually refers to grades of stainless steel such as 410, 420 and 431, with a chromium content of about 13% and 17% respectively. Due to the lack of sufficient ductility of this type of steel, and the pier stress cracks are very sensitive and weldable during the manufacturing process, the use is limited and has become a less concerned class of materials in stainless steel clusters.
To overcome the above shortcomings, at the end of the 50s, the Swiss introduced the concept of soft martensite. * The original purpose is to improve the welding performance of the turbine generator impeller. By reducing the content (* high carbon content is 0.07%) and increasing the nickel content (3.5%~4.5%), a series of new alloys have been developed. These alloys have high tensile strength, good ductility and improved welding properties.
With the advancement of smelting technology, AOD/VOD torch technology is widely used in the refining of stainless steel, and the high carbon content of such alloys is reduced from 0.07% to 0.05% and 0.03%. After people’s unremitting efforts, the carbon content is further reduced, and the alloy composition is further optimized, the comprehensive mechanical energy of stainless steel is improved, the corrosion resistance is good, especially the welding performance is significantly improved, forming a new super martensitic stainless steel series, becoming a dazzling series in stainless steel clusters, which has received widespread attention.
6. Application prospect of supermartensitic stainless steel
In addition to the characteristics of traditional martensitic stainless steel, which can be used in pumps, compressors, valves and other machining purposes, super martensitic offshore pipes have been successfully developed, meeting the requirements of offshore oil and gas companies for seamless pipes and transportation pipelines for processes, and becoming a new member of marine steel.
NAM Oil & Natural in the Netherlands has decided to modernize its wet gas processing facility at its natural gas field in Groningen in northern Holland, including an overhaul of 30 spherical tanks and the replacement of all pipelines, all of which are supermartensitic 13Cr stainless steel.
Oman’s LNG project requires laying tens of kilometers of pipelines, also using supermartensitic stainless steel. Egypt and Nigeria are also working on similar projects.
In addition, plants such as hydropower, mining equipment, chemical equipment, food industry, transportation and high-temperature pulp production are also potential application areas.
7. The economy of super martensitic stainless steel
Before the success of supermartensitic stainless steel, duplex stainless steel was often used for many areas where stainless steel was used, especially in environments containing carbon dioxide, or corrosive media containing carbon dioxide and hydrogen sulfide, and some special components even required the use of ultra-duplex stainless steel. Now, people are increasingly using super martensitic stainless steel to partially replace duplex stainless steel and super duplex stainless steel, which in addition to super martensitic stainless steel still has high strength, good corrosion resistance and good impact toughness at -40 °C in some corrosion environments, the main reason is that super-martensitic stainless steel is more economical than duplex stainless steel.
First of all, under the premise of equal weight and corrosion resistance, the use of super 13Cr stainless steel is about 30% cheaper than the use of duplex stainless steel.
Secondly, the strength of ultra-13Cr stainless steel is much higher than that of duplex stainless steel, so the wall thickness of parts made of ultra-13Cr stainless steel (such as tees, elbows, conveyor pipes and branch pipes) can be thinned, and the cost is reduced by 10%~15%. In total, compared with duplex stainless steel, the total cost of super 13Cr stainless steel is reduced by 35%~40%. Such a drastic cost reduction makes it impossible to ignore its use in increasingly competitive industries, such as the oil and gas industry.
8. Conclusion
(1) Super martensitic stainless steel has good mechanical properties, the typical yield stress б0.2 is 550~850MPa, the tensile strength is 780~1000MPa, and has good impact toughness and ductility.
(2) The microstructure of supermartensitic stainless steel is low-carbon tempered martensite. Depending on the amount of nickel content and the difference in heat treatment conditions, the structure of some supermartensitic stainless steels may have partially refined and dispersed residual austenite and a little бferrite.
(3) Because the carbon content is lower than that of traditional martensitic stainless steel, and the appropriate amount of nickel and molybdenum is added to the steel, the corrosion resistance is improved.
(4) The welding performance of super martensitic stainless steel is better than that of traditional martensitic stainless steel, and welding can be carried out by conventional welding process. Low-carbon and low-nitrogen supermartensitic stainless steels can be used in the welded state, and post-weld heat treatment can be applied if necessary to obtain lower hardness and better toughness.
(5) Super 13Cr stainless steel is a new type of economical and applicable steel, and its use cost is 35%~40% lower than that of duplex stainless steel, so that this type of steel has a strong competitive advantage in the stainless steel family, and the application prospects are very broad.
Petrochemical and chemical industry
The corrosion environment of petrochemical and chemical industry is characterized by high reaction temperature, and the medium often contains high or medium concentration of chloride, which is easy to induce stress corrosion fracture of stainless steel. In this field, not only duplex stainless steels are used, but also super duplex stainless steels.
Chlorinated olefin production plant
The corrosion environment of oxychlorination method to produce vinyl chloride is quite harsh, various grades of duplex stainless steel are used to manufacture containers, heat exchangers and piping systems in the device, * harsh corrosion conditions are contact with hydrochloric acid, super duplex stainless steel is mostly used in the manufacture of key equipment, such as oxychlorination reactors, HCl coolers and vinyl chloride towers.
Production equipment for organic acids such as acetic acid
Duplex stainless steel has good corrosion resistance in organic acids, often add various salt catalysts in acetic acid production, even super duplex stainless steel is not resistant to the corrosion of chloride catalysts containing Fe3+, Cu2+ oxidizing ions, but in the use of reducing catalysts such as MnCl2 Among the acetic acid of MnAC (manganese acetate), super duplex stainless steel SAF2507 has good corrosion resistance. In the production of dimethyl terephthalate, the use of NaBr and MnAc catalyst, acetic acid in the process is also a double product (its recovered concentration can reach 90%, 125C), the use of 904L and 825 alloy are not very ideal, super duplex stainless steel Zeron100 and SAF2507 can be used as ** cooler and catalyst regeneration equipment.
In the formaldehyde production medium, SAF2507 steel has good corrosion resistance, better than 904L and high-alloy austenitic stainless steel containing 6% Mo, which can be used as a column and evaporator, and the conditions of use:
Pipeline: formaldehyde, trace formic acid, 200 °C, 0.8MPa
Shell range: steam, 230 °C, 0.7MPa
The typical media conditions of chlorine dioxide bleaching cylinder are pH6~7,70°C, ≥600ppmCl-, 300ppmClO3– and 20~200ppmClO2. In this medium, although UR45N (SAF2205) is better than 316L steel, it still has hole erosion, * preferably using super duplex stainless steel UR52N+, SAF2507, etc., which can be comparable to austenitic stainless steel containing 6% Mo.
Super duplex stainless steel has also been used in the oil and gas industry, such as UR52N+ has been used in oil and gas collection pipes and transmission pipelines in North Sea oilfields, SAF2507 steel for oil and gas well production in Alaska, North Sea, Gulf of Mexico and other places, offshore oil production platform facilities and oil and gas transmission pipelines, etc., mostly used in harsh acid environments without corrosion inhibitors.
At present, as far as hot seawater steel is concerned, most of them use 25Cr-3Mo type duplex stainless steel, of course, super duplex stainless steel has a better use effect, such as SAF2507 steel made of seawater heat exchanger, the shell is butane containing a small amount of fluoride (inlet temperature 80 °C, outlet temperature 30 °C), the pipeline is seawater, the process temperature may rise from 40 °C to 80 °C, titanium tube because of F– resistance Corrosion, can only be used for 3 months, SAF2507 after 3 years of use has not found corrosion. The pipeline system on the North Sea oil production platform uses Zeron100 steel, the seawater outlet temperature can reach 60~65 °C, and no corrosion problem has been found, and when using 6% Mo austenitic stainless steel, stress corrosion fracture will occur at this temperature.
In recent years, in order to solve the problem of microbial corrosion, hypochlorite** is often used, and batch or continuous seawater is chlorinated. For example, when continuously chlorinating seawater, the residual chlorine content of 0.1~0.2ppm is enough to kill microorganisms. Chlorinated seawater is more corrosive than natural seawater, increasing its redox potential, making metal materials more sensitive to pore and crevice corrosion. SAF2507 steel is used in a heat exchanger with chlorinated seawater as the cooling medium, and the outlet temperature of the seawater side can reach 45~50 °C.
Energy and environmental protection industry
In order to reduce the atmospheric pollution caused by the combustion of coal in power plants and prevent acid rain, the FGD (flue gas desulfurization) method has been widely used in North America, Europe and Japan to remove sulfur from combustion gases since the late 80s. Some important components in FGD devices, such as vessels, fans (blades), agitators, pumps, valves and centrifuges, are required to be resistant to chloride corrosion, as well as harsh abrasive abrasive and slurry abrasion corrosion. Duplex stainless steels and super duplex stainless steels meet corrosion resistance and engineering economy requirements under certain media conditions.
The FGD plant of the two power stations of RASF in Germany uses a 2 °C aqueous solution of Na3SO50 to absorb SO2 in the flue gas, and the equipment in the evaporation process uses German 1.4462 (00Cr22Ni5Mo3N) duplex stainless steel, but pay attention to controlling the content of halogen ions, Cl– ion * high value is 2000mg/L. The plant’s duplex stainless steel equipment is in good condition after 3 years of use.
Another process commonly used in North America and Europe is the lime/limestone process. The process is to wash the gas containing SO2 with lime or limestone slurry, due to the incomplete reaction of the contaminated gas with the solution in some areas of the scrubber, the condensation of acids combines with chloride and fluoride, and constitutes a very harsh corrosive environment under the action of temperature. Of course, with different process parameters, fuel types and chloride ion content in water, the harshness of the corrosive environment is also different, so the selection of equipment is also different.
Typical media conditions are: 40~80 °C, 5>pH>3, about 0.1% ~ 0.5% chloride, about 500ppm fluoride, if there is sedimentation, the conditions will be more harsh, and due to local acidification and concentration phenomenon, pH may drop to 1, or even 0, and the concentration of chloride increases to 5% ~ 10%, and the concentration of fluoride increases to 1000ppm.
Under such conditions, since 1986, super duplex stainless steel UR52N+ (00Cr25Ni6.5Mo3.5CuN) has been used to manufacture scrubbers and absorbers, including some other internals and linings and other equipment, including the use of ordinary duplex stainless steel to manufacture centrifugal fan blades, expansion junctions and dampers for flue gas pipes, water supply logistics heat exchangers for wastewater treatment.
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