New High Toughness Ultra-high Strength Steels: Effects of Composition and Heat Treatment on Strength and Toughnes

There are currently available only two types of ultra-high strength martensitic non-stainless steels that are of high toughness. They are maraging steels and some secondary hardening steels. Secondary hardening steels of high toughness include HY180, AF1410 and AerMetRTM 100. Those alloys are relatively expensive due to high cobalt and nickel contents. The ultimate objective of this work is to further develop a new type of ultrahigh strength, high toughness steel which has lower cost than current high toughness steels such as AerMetRTM 100 due to the elimination of cobalt and the reduction of nickel content in the steel. Based on previous research, a new steel based on modifying hot work die steels such as H-11 was proposed. The composition of this base steel in wt. % is 0.38C-4.5Cr-2Mo-0.5W-0.5V. Norwood examined the effects of nickel content on the strength and toughness of this base steel but both the strength and toughness of this new experimental steel are below those of AerMetRTM 100. This work builds on the results of Norwood and has two objectives. The first objective is to understand the effects of nickel on strength and toughness. The second objective is to further improve the strength and fracture toughness of this steel focusing on the following approaches. They are increasing the chromium content, investigating the effects of austenitizing treatment, examining the effects of martempering on strength and toughness of this steel, and increasing the silicon content.;In the study of the effects of nickel content, the microstructure of two heats containing 0 and 5 wt. % nickel made in Norwood's work were examined using transmission electron microscope. Fine precipitates of M2C and MC type carbides were observed in both steels. No significant difference were detected for the fine precipitates between the two steels. Stringer-like or sheet-like inter-lath carbides were detected for both steels upon tempering. This suggests that nickel does not have an effect on the morphology of the inter-lath carbides. In addition, the effects of inclusions were examined by comparing the toughness of the newly made nickel-free heats with the old Berkeley nickel-free heats which contain different types of inclusions for the same heat treatment. Furthermore, it was found that the low toughness of the Ni-free heat is mainly caused by the formation of bainite on grain boundaries during the air cooling from the austenitizing temperature. Ni addition can greatly suppress the formation of bainite during the air cooling process and thus improve the toughness.;In the study of the effects of chromium content, two series of steels were prepared. In one series of steels the chromium content was varied at a nickel level of 3 wt. %. In the second the chromium was varied at a nickel level of 5 wt. %. In general, increasing chromium leads to an insignificant increase in strength for all the experimental alloys. On the other hand, increasing chromium content does not improve the toughness for 3 wt. % nickel alloys but leads to decreasing toughness for 5 wt. % nickel alloys. In addition, increasing chromium content increases the quench cracking susceptibility of the steels. The effects of austenitizing temperature and quenching rate from the austenitizing temperature were investigated using the alloys made to examine the effects of chromium. It was found that the strength in general increases with increasing austenitizing temperature but at the expense of toughness. On the other hand, using a slower cooling rate by air cooling results in higher strength than oil quenching. However, the toughness is lower for the specimens using air cooling than the oil quenching.;The effects of unconventional martempering treatment on the strength and toughness of the steels were examined with three purposes. They were avoiding precipitation of embrittling carbides or bainite on austenite grain boundaries during the quenching process for the improvement of toughness, reducing the susceptibility to quench cracking, and possibly obtaining additional precipitation strengthening through precipitation in the austenite during isothermal holding. In general the martempering heat treatment resulted in much higher toughness than when air cooling from the austenitizing temperature. The toughness obtained by martempering is also significantly greater than that achieved by oil quenching. In addition, almost no quench cracking occurred during the martempering process. However, the strength level of the heats using martempering treatment is lower than the strength achieved using conventional heat treatment. It was found that martempeirng treatment is more effective in terms of improving toughness when the Cr content is low and when the Ni content is high. (Abstract shortened by ProQuest.).