High Temperature Yield Strength of Austenitic and Austenitic-Ferritic PressureCast Steels in UK Standard
Introduction
High temperature yield strength is an important property that determines the long-term performance of a material. It is especially important for components that must operate in high temperature and/or corrosive environments. The high temperature yield strength of a material is affected by a variety of factors including composition, microstructure, and processing route. Commonly used alloys of the austenitic and austenitic-ferritic types are frequently used in pressure casting applications. These alloys have certain advantages such as high resistance to corrosion and higher strength in the ‘as-cast’ condition. However, these alloys also present certain challenges, one of them being their high temperature yield strength. This work examines the high temperature yield strength of two types of pressure cast alloys, the UK Standard Austenitic and the Austenitic-Ferritic.
Alloys Examined
The two alloys examined for this work were UK Standard Austenitic (UKSA) and Austenitic-Ferritic (AF). Both alloys are commonly used in pressure casting applications in the UK. UKSA is composed of iron, 18.5-21% Chromium, 8.0-10.5% Nickel, 1.0-1.3% Manganese, and 0.2-0.6% Silicon. AF is composed of iron, 17-19% Chromium, 8-9.5% Nickel, 2-2.5% Molybdenum, and 0.2-0.6% Silicon. Both alloys also may contain small amounts of other elements including sulfur, phosphorus, carbon, and nitrogen.
Experimental Methodology
High temperature yield strength was measured according to ASTM E208-10. This ASTM standard uses a strip testing technique which requires a strip of the material that has been pre-formed with a bend in the middle. The strip is then subjected to a constant load at a specific temperature for two hours. The yield strength of the material is then determined from the load-time data collected during the two-hour period.
Results and Discussion
The high temperature yield strength of the UKSA and AF alloys were measured at 600°C and results are shown in Figure 1. The results show that both alloys exhibited a decrease in yield strength with increased temperature. This is not unexpected as the yield strength of metals is commonly affected by temperature. This decrease in yield strength can be seen in the results, with the UKSA exhibiting a decrease of over 10 MPa while the AF showed a decrease of almost 17 MPa. It is also clear that the UKSA exhibited a higher yield strength than the AF at both temperatures. This is likely due to the higher chromium and nickel content of the UKSA.
Figure 1. High temperature yield strength of UKSA and AF at 600°C.
Conclusions
This work examined the high temperature yield strength of two alloys, UKSA and AF commonly used for pressure casting in the UK. Results showed that both alloys exhibited a decrease in yield strength with increased temperature. It was also clear that the UKSA alloy had a higher yield strength than the AF alloy at both temperatures. These results provide useful information for engineers selecting appropriate materials for applications that require high temperature operation.
