Hot ductility of an austenitic and a ferritic stainless steel

Abstract

The influence of strain rate and grain size on the hot ductility of an austenitic and aferritic stainless steel has been examined. Samples were cooledfrom the austenitising temperature to temperatures in the range 1000 to 700°C and tensile tested at strain rates in the range 10-1 to 10 -4 S -1. The austenitising temperature was varied to give two grain sizes, coarse, 600 11mand fine, I"V 30 11m.For both steels, ductility was excellent at fine grain size throughout the temperature range and strain rates examined. The ferritic and austenitic stainless steels both gave ductility troughs at the coarse grain size, but the trough was favoured by higher strain rates in the ferritic steel and lower strain rates in the austenitic steel. The poor ductility was related to the presence of precipitation, mainly at the grain boundaries; this being FeTi phosphides in the case oftheferritic stainless steel and coarse chromium carbides in the austenitic steel. Grain boundary sliding was the major mode of intergranular failure in the austenitic steel while normal microvoid coalescent failure controlled ductility in theferritic stainless steel.MST/3474

Influence of strain rate on hot ductility of austenitic stainless steel slab

“…When strain rates are in the range of 10 24 -10 21 s 21 , the formation of chromium carbides along austenite grain boundaries causes the ductility trough at lower strain rates in austenitic stainless steel, and increasing strain rate causes the ductility to improve. 5 Similar results are found in V microalloyed steel in the same strain rate range, but the precipitation is V(C,N). 6 Ductility increases with strain rate mainly due to the short time available for precipitation formation as well as for intergranular void growth and coalescence.…”

Hot ductility of high Al TWIP steels containing Nb and Nb-V

“…The two types of fracture are subject to entirely different mechanisms. For example from the previous work, when comparing the hot ductility of fully austenitic with fully ferritic structures in the temperature range under examination [26], the former was mainly inter-granular having in this case precipitates at the boundaries and the latter trans-granular. Austenite recrystallises when deformed at elevated temperatures, so that the amount of grain boundary sliding and dynamic recrystallisation control the fracture process.…”

Understanding the high temperature side of the hot ductility curve for steels

“…As with steels which undergo the ϒ/α transformation on straightening, it is the hot ductility of unrecrystallised austenite which is important in controlling the cracking susceptibility. In austenitic steels, as with HSLA, TRIP and dual phase steels which undergo transformation, increasing the strain rate and refining the grain size improves ductility whereas precipitation impairs the ductility particularly when the precipitates are situated at the austenite grain boundaries [13]. The only difference is that there is no ferrite present when straightening, so that the deformation induced thin film of ferrite (DIF) surrounding the austenite grains which is present in the HSLA steels and widens and deepens the ductility trough is absent.…”

“…When precipitation takes place in austenitic steels, the curves dip down, Figure 7, reach a minimum value for RA and instead of remaining relatively flat, then rise again [13]. The minimum ductility in this particular case is due to the precipitation of chromium carbides at the boundaries and within the matrix.…”