Structure-property relations and the design of Fe-4Cr-C base structural steels for high strength and toughness

Abstract

Some design guidelines for improving strength-toughness combinations in medium carbon structural steels are critically reviewed. From this, quaternary alloy development based on Fe/Cr/C steels with Mn Jr Ni additions for improved properties is·described. Transmissjon electron microscopy and X-ray analysis reveal increasing amounts of retained austenite in these alloys with r~n content up to 2 w/o and Ni additions at 5 w/o after quenching from 1100° C. A corresponding improvement in toughness properties is also found. Grain refining results in a further increase in the amount of re--,.tained austenite. In addition, the excellent combinations of strength and toughness in these quaternary alloys are attributed to the production of dislocated lath martensite from a homogeneous austenite phase free from uridi sso 1 ved all oy' carbi des.The question of thermal instability of retained austenite following tempering is considered in detail and it is shown that the decomposition of retained austenite is closely related to the ease of n~cleation and growth of cementite. Thus, graphitizing alloying elements such as Ni are beneficial in postponing tWe decomposition of retained austenite.

Sulfide stress cracking of nickel-containing low-alloy steels

“…However, as shown by Grange (1973), nickel has a marked effect on hardenable diameter when its concentration is above 1-wt%. Moreover, nickel together with manganese, is the only alloying element that has the unique ability of simultaneously increasing hardenability, strength, and toughness (Davis, 2001;Narasimha Rao & Thomas, 1980) with little penalty on weldability, as explained in Section 3.5. Nickel does not form carbides and therefore delays proeutectoid ferrite, pearlitic, and bainitic reactions (Siebert et al, 1977).…”

“…However, retained austenite is necessary for extreme lowtemperature applications where DBTT as low as -200°C are required (Campbell, 2008). In this regard, steels with 5-wt% Ni can be heat treated to yield a microstructure consisting of lath martensite with fine retained austenite films in prior austenite grain boundaries ( Narasimha Rao & Thomas, 1980) or austenite with ferrite and tempered martensite (Sarno, Bruner, & Kampschaefer, 1974). The effect of retained austenite is therefore beneficial in terms of fracture toughness because the entire "absorbed energy vs. temperature" curve shifts upward toward higher energies, accompanied by a decrease in DBTT (Narasimha Rao & Thomas, 1980).…”

“…In this regard, steels with 5-wt% Ni can be heat treated to yield a microstructure consisting of lath martensite with fine retained austenite films in prior austenite grain boundaries ( Narasimha Rao & Thomas, 1980) or austenite with ferrite and tempered martensite (Sarno, Bruner, & Kampschaefer, 1974). The effect of retained austenite is therefore beneficial in terms of fracture toughness because the entire "absorbed energy vs. temperature" curve shifts upward toward higher energies, accompanied by a decrease in DBTT (Narasimha Rao & Thomas, 1980). Steels for ultralow-temperature and cryogenic applications conform to, e.g., ASTM A320/A320M, A333/A333M, A352/A352M, A353, A553, A645, and shall meet minimum CVN impact energy requirements.…”

“…Interlathretained austenite in UNS G434000 (SAE/AISI 4340) also increases fracture toughness (Zhirafar et al, 2007). Proposed mechanisms for the improved fracture toughness with retained austenite include promotion of crack branching, crack blunting due to plastic flow in retained austenite, prevention of interlath boundary carbide precipitation, and transformation-induced plasticity, where retained austenite transforms to martensite at the crack tip, relieving stress concentration (Malakondaiah et al, 1997;Narasimha Rao & Thomas, 1980).…”

Medium carbon steel alloy design for wear applications

“…The results of sliding and abrasive wear tests are shown in Figures [8][9][10][11][12][13] and in Table 2 and 3 along with the respective mechanical properties.…”