Mechanical Properties of AISI 347 (0Cr18Ni11Nb) at Low Temperatures
The influence of temperature on a material’s mechanical properties is well known, with different materials displaying different behaviors depending on temperature. Low temperatures, particularly in cryogenic environments, can cause significant aberrations from the traditional mechanical properties seen at room temperature. Consequently, if these materials are to be employed in cryogenic applications, these anomalous mechanical behaviors need to be examined, understood, and accounted for in engineering designs. One such material commonly employed in cryogenic applications is the stainless steel alloy AISI 347 (0Cr18Ni11Nb), which is composed of 0.08% Carbon, 18% Chromium, 11% Nickel, and trace amounts of other elements, such as Molybdenum and Niobium. In this article, we will examine the mechanical properties of AISI 347 when exposed to low temperatures.
The mechanical properties of AISI 347 tend to decrease greatly with decreasing temperature. At ambient temperature, AISI 347 is a very strong material, with Ultimate Tensile Strength (UTS) ranging from 85-190 MPa, a yield strength ranging from 30-80 MPa, a maximum hardness of about 89 HV, and a ductility ranging from 18-35%. However, as the temperature decreases towards cryogenic temperatures, fluctuations in chemical composition and microstructure will cause the UTS, Yield Strength, hardness, and ductility to drop significantly.
When subjected to temperatures as low as −196 °C (77 K), AISI 347 displays a UTS of 14 MPa, a Yield Strength of 8 MPa, a hardness of 66 HV, and a ductility of 11%. As the temperature continues to decline, the UTS of AISI 347 will continue to decrease. At −273 °C (0 K), the UTS is estimated to reach a minimum of 4.4 MPa, and is expected to remain relatively constant until a temperature of about 0.2 K. After this, the UTS will begin to increase slowly until reaching a value of 10 MPa at 1 K.
Generally, the UTS of AISI 347 will remain relatively constant until a temperature of about 0.2 K, with the exact change in UTS dependent upon temperature, prior exposure, and prior strain levels. As the temperature continues to decline below 0.2 K, the UTS will decrease significantly. The Yield Strength, Maximum Hardness, and Ductility will also decrease in this range.
Visco-elastic creep is an issue with all materials at low temperatures, and AISI 347 is no different. Visco-elastic creep involves the gradual reduction in a material’s mechanical properties over an extended period of time, as a result of repeated stresses and strain cycles. At low temperatures, the visco-elastic creep rate of AISI 347 is relatively low, but is still significantly greater than at room temperature. Consequently, when selecting AISI 347 for cryogenic applications, it is vital to ensure the mechanisms of the system will not cause stress or strain cycles which would eventually cause parts to fail.
At low temperatures, AISI 347 displays a significant decrease in mechanical properties when compared to room temperature. As a result, in order for engineers to design safe, reliable structures for cryogenic applications, it is important to be aware of and account for the anomalous mechanical properties of AISI 347 when exposed to low temperatures. For temperatures below 0.2 K, the UTS of AISI 347 may decrease significantly, and visco-elastic creep must be taken into account in order to ensure a reliable system. With this knowledge, engineers can more effectively select materials and perform safe and reliable engineering designs for cryogenic applications.
