A novel heat-treating process, quench and partitioning (Q&P), has been proposed as a fundamentally new way to produce martensitic microstructures containing retained austenite. The two-step process hypothesizes carbon enrichment of the austenite by decarburization of the martensite. Significant amounts of retained austenite have been measured in the final microstructure, although evidence for transition carbide formation in the martensite also exists. The mechanical properties obtained via Q&P are reported for a CMnAlSiP steel after intercritical annealing for A50 specimens. Tensile strength/total elongation combinations, ranging from 800 MPa/>25 pct to 900 MPa/20 pct to 1000 MPa/10 pct, indicate that Q&P is a viable way to produce high strength steel grades with good ductility. The instantaneous strain hardening of Q&P steels shows a significant dependence on the partitioning conditions applied. Lower partitioning temperature (PT) leads to continuously decreasing instantaneous n-values with strain, similar to the strain hardening behavior observed for dual-phase (DP) steels, whereas higher PTs for the same partitioning time increase the strain hardening significantly. After an initial increase, the observed n-values remain high up to considerable amounts of strain, resulting in similar strain hardening behavior observed for austempered transformation-induced plasticity (TRIP) grades. Assessment of the mechanical stability of the retained austenite indicates that the TRIP effect is effectively contributing to the increased strain hardening as function of strain.

Effect of Retained Austenite Stabilized via Quench and Partitioning on the Strain Hardening of Martensitic Steels

This paper presents the results of a systematic study of the effect of carbon, manganese, phosphorus, silicon, nickel, chromium, molybdenum, and vanadium on the hardness of martensite in low to medium carbon steels tempered for one hour at 100°F (56°C) intervals in the range 400 to 1300°F (204 to 704°C). Results show that the as-quenched hardness depends solely on carbon content. On tempering, the effect of carbon on hardness decreases markedly with increasing tempering temperature. Studies of carbon-0.5 manganese steels showed that the incremental increase in hardness from 0.5 pct manganese after a given tempering treatment was independent of carbon content. Based on this result, studies of the effects of the other alloying elements were made using a 0.2 or 0.3 pct carbon, 0.3 to 0.5 pct manganese steel base composition. The hardness of the resulting tempered martensite was assumed to be due to a given alloy addition, and when two or more alloying elements were added, their effects were assumed to be additive. Each of the seven alloying elements increased the hardness of tempered martensite by varying amounts, the increase being greater as more of each element was present. Nickel and phosphorus have substantially the same effect at all tempering temperatures. Manganese has essentially the same hardening effect at any temperature in the range 700 (371°C) to 1300°F; silicon is most effective at 600°F (316°C), chromium at 800°F (427°C), molybdenum at 1000 to 1100°F (538 to 592°C), and vanadium at 1200°F (649°C). Using the data obtained, a procedure is established for calculating the hardness of tempered martensite for carbon and alloy steel compositions in the range studied and for any combination of tempering time and temperature.

Hardness of tempered martensite in carbon and low-alloy steels

Microstructures and hardness of as-quenched martensites (0.1-0.5%C)

Thermomechanical processing of advanced high strength steels

Mechanical properties and microstructural features of AISI 4340 high-strength alloy steel under quenched and tempered conditions

Tempering of martensitic steels involves the segregation of carbon, the precipitation of carbides, the decomposition of retained austenite, and the recovery and recrystallization of the martensitic structure Because these several reactions overlap and occur on such a fine scale, it is only recently that our knowledge of the resulting structures has become reasonably complete Our present understanding of the processes involved in the tempering of iron-carbon martensites and how they are affected by alloying elements is reviewed

G. R. Speich

Papers

Tempering of steel