The 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) martensitic stainless steels are also known as the 440 series of stainless steel. 7Cr17, 8Cr17 and 11Cr17 are differentiated by their higher chromium content levels (7Cr17 is allowed up to a maximum of 9% chromium, while the 8Cr17 and 11Cr17 are allowed up to a maximum of 12% and 17% chromium respectively).
It is a general characterization of the 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) martensitic stainless steels that they have good corrosion resistance, higher hardness and strength together with good mechanical properties and very good workability. These qualities make the 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) martensitic stainless steels suitable for a wide range of applications.
The mechanical properties of the 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) martensitic stainless steels at high temperature are determined by their tempered martensitic microstructure and hardness. As they are single-phase steels, they are brittle at high temperatures and must be well-preserved with proper overaging treatment.
At room temperature, the 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) martensitic stainless steels can achieve excellent strength and hardness levels. With 7Cr17, the ultimate tensile strength (UTS) is usually about 700 MPa, for 8Cr17 it is usually 800-900MPa, and for 11Cr17 it is usually 1000-1200MPa. Thermal treatment can further increase the UTS and hardness of these martensitic stainless steels.
At elevated temperatures, the mechanical properties of the 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) martensitic stainless steels may become nonlinear due to the presence of the martensitic microstructure and the hardness decrease associated with it. Thus, the yield strength (YS) and ultimate tensile strength (UTS) decrease while the elongation (EL) and the elastic modulus (EMS) increases with increasing temperature. This effect becomes more pronounced at temperatures exceeding 600°C.
At temperatures above 600°C, the YS and UTS of 7Cr17 are expected to decrease significantly, depending on the service environments. Based on data available in the literature, the UTS of 7Cr17 is expected to decrease by 10-20% at 500-600°C and by 40-50% at 700-800°C. Similarly, the UTS of 8Cr17 is expected to decrease by 20-25% at 500-600°C and by 50-60% at 800-900°C. Finally, the UTS of 11Cr17 is expected to decrease by 25-30% at 600-700°C and by 60-70% at 1000-1100°C.
The EMS of 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) is expected to increase from room temperature to temperatures higher than 500°C. Above 1000°C, however, the EMS begins to decrease due to the formation of a more disordered ordered substructure, which has a lower modulus of elasticity than the martensitic ordered substructure.
The ductility or EL of the 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) martensitic stainless steels is sensible to high temperatures. Data from literature suggests that the EL increases up to temperatures higher than 700°C, but then starts to decrease above 900°C.
In conclusion, the 7Cr17 (440A), 8Cr17 (440B) and 11Cr17 (440C) martensitic stainless steels are capable of achieving good mechanical properties at room temperature and have good ductility up to temperatures higher than 700°C. However, the mechanical properties of these stainless steels begin to significantly decrease at temperatures higher than 600°C. These high-temperature properties need to be carefully considered when designing components from these steels.
