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» Technical » 1.25Cr-0.5Mo | 1.25Cr-0.5Mo-Si

1.25Cr-0.5Mo | 1.25Cr-0.5Mo-Si

September 21, 2020

Chemical Composition & Characteristics

ASTM A387 Gr.11 (1.25Cr-0.5Mo-Si) plates

ASTM A387 Gr.11 Class 2 (1.25Cr-0.5Mo-Si steel) plates for boiler construction, produced by WUSTEEL.

The 1.25Cr-0.5Mo (also known as “1-1/4Cr-1/2Mo”) steel is a typical low alloy heat-resistant steel of the Cr-Mo steel family. It is widely used for applications at elevated temperatures. The figures “1.25” and “0.5” denote the nominal chemical composition of chromium and molybdenum alloying elements respectively. The chromium provides improved oxidation and corrosion resistance, and, the molybdenum increases creep strength at elevated temperatures. In ASME Codes, the 1.25Cr-0.5Mo generally refers to castings. Various wrought 1.25Cr-0.5Mo steels are furnished with necessary addition (0.5~1.0%) of silicon element, thus, are designated as “1.25Cr-0.5Mo-Si“. The silicon increases the elevated-temperature strength of steel. It also increases the resistance to scaling of the low-chromium steels in air at elevated temperatures. The 1.25Cr-0.5Mo steel may be furnished in annealed, normalized and tempered, or quenched and tempered condition.

Product Forms, Standards & Grades

Product FormsStandards & Grades
CastingsASTM A217 WC6
ASME SA-217 WC6
Cast PipesASTM A426 CP11
ASME SA-426 CP11
BarsASTM A739 B11
ASME SA-739 B11
ForgingsASTM A182 F11
ASME SA-182 F11
ASTM A336 F11
ASME SA-336 F11
Seamless &
Welded Fittings
ASTM A234 WP11
ASME SA-234 WP11
Seamless TubesASTM A213 T11
ASME SA-213 T11
Welded TubesASTM A250 T11
ASME SA-250 T11
Seamless PipesASTM A335 P11
ASME SA-335 P11
Welded PipesASTM A691 1.25Cr
ASME SA-691 1.25Cr
Forged PipesASTM A369 FP11
ASME SA-369 FP11
PlatesASTM A387 11
ASME SA-387 11

The 1.25Cr-0.5Mo (1.25Cr-0.5Mo-Si) steels can be provided in a variety of product forms in accordance with corresponding ASTM/ ASME standard specifications.

Physical Properties

The density of 1.25Cr-0.5Mo steel is 7750 kg/m3, and the Poisson’s ratio is 0.30. The melting temperature range is 2600°F-2800°F. The typical values of thermal expansion (instantaneous, mean, and linear), thermal conductivity, thermal diffusivity, and modulus of elasiticity of the 1.25Cr-0.5Mo steel are listed in tables below. These values are listed as a function of temperature from 20°C to as high as 825°C.

Temp. °CABC
2011.511.50
5012.011.80.35
7512.311.90.65
10012.712.10.97
12512.912.31.3
15013.212.41.6
17513.512.62.0
20013.812.72.3
22514.012.92.6
25014.313.03.0
27514.613.23.4
30014.913.33.7
32515.113.44.1
35015.413.64.5
37515.713.74.9
40015.913.85.3
42516.114.05.7
45016.414.16.1
47516.514.26.5
50016.714.46.9
52516.814.57.3
55016.914.67.7
57517.014.78.2
60017.014.88.6
62517.114.99.0
65017.115.09.4
67517.115.19.9
70017.115.110.3
72517.115.210.7
75017.215.311.1
77517.415.311.6
80017.715.412.0
82518.115.512.5
*Thermal expansion for 1.25Cr-0.5Mo steels: Coefficient A is the instantaneous coefficient of thermal expansion ×10−6(mm/mm/°C). Coefficient B is the mean coefficient of thermal expansion ×10−6(mm/mm/°C) in going from 20°C to indicated temperature. Coefficient C is the linear thermal expansion (mm/m) in going from 20°C to indicated temperature. All values are extracted from Material Group 1 of ASME BPVC Section II Part D Table TE-1

Temp. °CTCTD
2041.011.87
5040.811.47
7540.711.16
10040.610.88
12540.510.60
15040.410.33
17540.310.08
20040.19.82
22539.89.57
25039.59.32
27539.19.07
30038.78.82
32538.38.57
35037.88.32
37537.38.06
40036.87.81
42536.37.55
45035.87.29
47535.37.03
50034.86.77
52534.46.50
55033.96.24
57533.45.97
60032.85.69
62532.25.42
65031.65.14
67530.74.78
70029.14.37
72527.63.83
75026.72.69
775-1.26
800-5.06
825-7.74
*Nominal coefficients of thermal conductivity and thermal diffusivity for 1.25Cr-0.5Mo steels: TC is the thermal conductivity, W(m·°C), and TD is the thermal diffusivity, 10−6m2/sec. The specific heat [J/(kg·°C)] can be calculated by the equation: TD = TC / (Density × specific heat). All values are extracted from Material Group C of ASME BPVC Section II Part D Table TCD.

Temp.E
-200218
-125213
-75210
25204
100200
150197
200193
250190
300186
350183
400179
450174
500169
550164
600157
650150
700142
*Modulus of elasticity E (×103 MPa) for given temperatures (°C) of 1.25Cr-0.5Mo (1.25Cr-0.5Mo-Si) steels. All values are extracted from Material Group C of ASME BPVC Section II Part D Table TM-1.

Creep Strength & Rupture Strength

creep & rupture strength of 1.25Cr-0.5Mo

Illustration of creep strengths and rupture strengths of 1.25Cr-0.5Mo steels, compared to that of 1.0Cr-0.5Mo.

Due to the improved creep strength, 1.25Cr-0.5Mo steels can be used at temperatures as high as 649°C [1200°F]. It has comparable stress-rupture and creep properties at elevated temperatures as that of 1.0Cr-0.5Mo alloy steels.

Air Hardening Characteristics

Temp.
°F
[°C]
Hardness,
HB
Annealed156
1400
[760]
149
1500
[815]
156
1600
[870]
179
1650
[900]
187
1750
[955]
229
1850
[1010]
223
1950
[1065]
212

Hardness is one of the key parameters of the heat-resistant steel. In order to determine the hardness values after air cooling, several pieces of round specimens (25 mm in diameter and 50 mm in length) were sliced longitudinally and heated. The hardness values are measured after air cooling from designated temperatures and presented in the table above.

Supply Chain

Due to the huge consumption in many elevated-temperature applications, a complete supply chain for the 1.25Cr-0.5Mo steel has formed in China’s steel industry. It means that this steel is 100% recyclable and full spectrum of starting materials are commercially available. Thus, we can not only supply 1.25Cr-0.5Mo steels in various product forms such as casting, forging, pipe, tube, fitting, valve part, and pressure vessel, but also reduce the cost to the minimum.

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