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Physical Properties of Martensitic Chromium Stainless Steels 7Cr17 (440A), 8Cr17 (440B), 11Cr17 (440C)
Martensitic chromium stainless steels are defined as a group of alloys containing a martensitic microstructure, up to 30 percent chromium, and up to 40 percent carbon. Martensitic chromium stainless steel alloys are forged, machined, formed, or cold worked in order to produce specific components for industrial, naval, and aircraft applications. The most common alloys within the martensitic chromium series, 7Cr17 (440A), 8Cr17 (440B), and 11Cr17 (440C), are the focus of this article. This paper will discuss the physical properties of the martensitic chromium stainless steels, specifically related to their metallurgical composition, yield strength, tensile strength, and elongation.
Martensitic chromium stainless steels, such as 7Cr17 (440A), 8Cr17 (440B), and 11Cr17 (440C), are classified as standard chromium ferritic stainless steel alloys. The composition of each alloy was determined using elemental analysis and is represented in Table 1. Each alloy contains varying amounts of carbon and chromium, with a balanced combination of the elements necessary to promote a martensitic microstructure.
Table 1 – Metallurgical Composition of Martensitic Chromium Stainless Steels
Alloy C Mn P S Si Cr Ni Mo 7Cr17 (440A) 0.6-0.75 ≤1.00 ≤0.040 ≤0.03 ≤1.00 16-18 ≤0.75 - 8Cr17 (440B) 0.75-0.95 ≤1.00 ≤0.040 ≤0.03 ≤1.00 16-18 ≤0.75 - 11Cr17 (440C) 0.9-1.20 ≤1.00 ≤0.040 ≤0.03 ≤1.00 16-18 ≤0.75 -
The physical properties of the martensitic chromium stainless steels, 7Cr17 (440A), 8Cr17 (440B), and 11Cr17 (440C) are highlighted in Table 2. As the carbon content of each alloy is increased, the overall strength of the material is increased. The yield strength, ultimate tensile strength, elongation, and hardness values increased as the carbon content increased. The highest yield strength (243 MPa) and ultimate tensile strength (566 MPa) were seen in the 11Cr17 (440C) alloy.
Table 2 - Mechanical Properties of Martensitic Chromium Stainless Steels
Alloy Yield Strength (MPa) Ultimate Tensile Strength (MPa) Elongation (%) Hardness (Brinell) 7Cr17 (440A) 147 420 21 240 8Cr17 (440B) 194 510 22 260 11Cr17 (440C) 243 566 21 282
The mechanical properties of each alloy are determined by the chromium content, carbon content, and their combination. The increased hardness and yield strength of the martensite chromium stainless steels are due to the presence of the martensitic microstructure. The martensitic microstructure contains martensite crystals with relatively low-dislocation density. This establishes a high strain hardening rate, allowing them to absorb large amounts of energy without being significantly deformed. The martensitic stainless steels can be subjected to stress concentration and still maintain the same strength properties in comparison to the bulk material.
In conclusion, martensitic chromium stainless steels, 7Cr17 (440A), 8Cr17 (440B), and 11Cr17 (440C), are defined as a group of alloys containing a martensitic microstructure, up to 30 percent chromium, and up to 40 percent carbon. The mechanical properties of each alloy are determined by the chromium content, carbon content, and their combination. The physical properties of the martensitic chromium stainless steels, specifically related to their metallurgical composition, yield strength, tensile strength, and elongation, are highlighted. The increased hardness and yield strength of the martensite chromium stainless steels are due to the presence of the martensitic microstructure, which has relatively low-dislocation density.
