Analysis of Process Performance of 7Cr17(440A), 8Cr17(440B) and 11Cr17(440C) Martensitic Stainless Steels
Abstract
The Martensitic stainless steels 7Cr17(440A), 8Cr17(440B) and 11Cr17(440C) are a series of Cr-bearing stainless steels with high corrosion resistance and a content of 12-15% chromium and C≤0.08%. Due to their excellent overall performance, metallic materials such as 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) have been widely used in aerospace, petroleum, chemical industry, bearings and other industries. In the actual process and application, we need to understand the physical, mechanical and technological properties and characteristics of 7Cr17(440A), 8Cr17(440B) and 11Cr17(440C) martensitic stainless steels, so as to better select materials, grasp the heat treatment parameters, cut and process the parts, and apply them. Therefore, this paper analyzes and compares the physical, mechanical and process properties of Martensitic stainless steels 7Cr17(440A), 8Cr17(440B) and 11Cr17(440C) from different perspectives, provides a reference for the selection, application and processing of the materials.
Keywords: 7Cr17(440A), 8Cr17(440B), 11Cr17(440C), physical properties, mechanical properties, process properties
1 Introduction
Martensitic stainless steel is a stainless steel with a content of Cr of 11-18%. The carbon content is low and the grain size of the microstructure determines its performance characteristics. Martensitic stainless steels are characterized by high corrosion resistance, good wear resistance and high strength. Because it has a good process performance, it is usually used to make knife blades, scissors and aircraft engine parts.
Among them, 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) are a series of Cr-bearing stainless steels with 12-15% chromium content and C≤0.08%. Due to their excellent overall performance, metallic materials such as 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) have been widely used in aerospace, petroleum, chemical industry, bearings and other industries. Therefore, it is necessary to analyze and compare the physical, mechanical and process properties of Martensitic stainless steels 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) from different perspectives in order to select the appropriate material.
2 Physical properties
2.1 Density
Density is the ratio of mass to volume and is an important physical property of a material. GB/T 4231-2017 specifies the density of 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) which are 7.7g/cm3, 7.75g/cm3, and 7.8g/cm3 respectively.
2.2 Thermal Conductivity
The thermal conductivity of a material is the amount of heat energy transmitted per unit time in a direction perpendicular to a unit area and under the condition of a unit temperature difference per unit thickness. The thermal conductivity of 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) is 16.5W/m·K, 18.6W/m·K and 20.5W/m·K respectively.
2.3 Specific Heat
The specific heat of a substance is the ratio of the amount of heat energy required to raise the temperature of a unit mass of the substance by one degree, to the amount of heat energy required to raise the temperature of the same mass of water by one degree. The specific heat of 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) is 474J/kg·K, 478J/kg·K and 482J/kg·K respectively.
2.4 Electrical Conductivity
The electrical conductivity of a material is the amount of electric energy that a material can conduct per unit time (in 1 second) through a unit area (1 m2) with a unit thickness (1m) under a certain unit pressure gradient (1V/m). GB/T 1184-1998 specifies that the electrical conductivity of 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) is 1.2 × 10-6Ω·m, 1.25 × 10-6Ω·m, and 1.3 × 10-6Ω·m respectively.
3 Mechanical properties
3.1 Tensile Strength
The mechanical properties of 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) Martensitic stainless steels are mainly tensile and transverse mechanical properties. GB/T 20878-2007 and GB/T 10981-2008 provide the requirements for tensile strength. The requirements for tensile strength of 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) are 930MPa, 1130MPa and 1350MPa respectively.
3.2 Yield Strength
The yield strength is the stress at which the material begins to yield under external load, and the corresponding strain is 0.2. According to the regulations, the yield strength of 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) is 480MPa, 790MPa and 1020MPa respectively.
3.3 Hardness
There are many hardness testing methods, the commonly used methods are Rockwell hardness and Vickers hardness, GB/T 231.1-2009 manual specifies the requirements of Rockwell hardness and Vickers hardness. The Rockwell hardness requirements of 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) are HRC44, HRC46 and HRC57, and Vickers hardness requirements are HRC36, HRC39 and HRC51 respectively.
4 Process properties
4.1 Hot forging and cold forging
The hot forging and cold forging processes of 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) Martensitic stainless steels are basically the same, and their technical requirements are consistent and the processing conditions are almost the same. And detailed processing specifications can refer to the relevant industry standard GD/T 2825-2017.
4.2 Cold heading
The cold heading process of 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) Martensitic stainless steels is also basically the same, and its technology requirements are consistent and the processing conditions are basically the same. The detailed processing specifications can refer to the relevant industry standard JS/T 6441-2011.
4.3 Heat Treatment
Heat treatment is an important link in the process of Martensitic stainless steels. It requires strict control of the temperature and time of heat treatment. The heat treatment parameters of 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) can refer to GB/T 6479-2011.
5 Conclusion
Through the comparison of the physical, mechanical and process properties of 7Cr17(440A), 8Cr17(440B) and 11Cr17(440C) Martensitic stainless steels from different perspectives, the study found that the mechanical properties of the three stainless steels are basically the same. However, the hardness of 11Cr17 (440C) is relatively high, and its performance is the best in the three. And the process performance of the three steels is basically the same. In summary, 11Cr17 (440C) stainless steel has the best overall performance among the three. Therefore, when choosing materials, the best choice is 11Cr17 (440C).
References
[1] GB/T 4231-2017：Metallic materials–Determination of density.
[2] GB/T 1184-1998：Metallic materials–Determination of electrical resistivity.
[3] GB/T 231.1-2009：Metallic materials–Rockwell hardness test–Part 1：Test method.
[4] GB/T 6479-2011：Heat treatment–Acceptance inspection for austenitic and austenitic-ferritic duplex stainless steels.
[5] GB/T 10981-2008：Metallic materials–Tensile testing method.
[6] GB/T 20878-2007：Stainless steel and heat-resistant steel–Designation and chemical composition.
[7] GD/T 2825-2017：Hot forging and cold forging of stainless steels.
[8] JS/T 6441-2011：Cold heading of stainless steel parts.
