Structure-property changes during hardening and tempering of new ultra high strength medium carbon low alloy steel

Abstract

not desirable, while an increase in the carbon level in conventional processing severely aVects the ductility, fracture A medium carbon low alloy steel, electroslag toughness and weldability of such a class of steel. 2 refined, modified AFNOR 15CDV6, has been Electroslag re ning (ESR) processing has provided an developed for satellite launch vehicle and alternative. 5-8 The ESR processed, with suitable inoculation, related applications. Conventionally processed ascast 15CDV6 alloy showed a 40% improvement in ductility (without electroslag refining) mostly bainitic and a marginal increase in yield strength. 5,7 Such impressive AFNOR 15CDV6 (with 0·15 wt-% carbon and gains were typically attributed to the complete eliminatioñ 3·5 wt-% other alloying elements) has a yield of inclusions above 12 mm in size. 5 It was even possible to strength of~800 MPa. Electroslag refining, double the carbon percentage (from 0·15 to 0·3 wt-%, Table 2) coupled with increased carbon (0·29 wt-% carbon, and thus improve the yield strength from 800 to~1300 MPa, but no change in percentage of other alloying without sacri cing the ductility of the original 15CDV6. 6,7 elements), increased the yield strength to aboutThe mechanical properties and chemical compositions of 1300-1400 MPa, without sacrificing ductility. The AFNOR 15CDV6 and ESR modi ed grades are listed in microstructure of the modified grade wasTables 1 and 2. The ESR modi ed 15CDV6 grade is now a martensitic. Martensite in the as hardened state promising candidate alloy in Indian satellite launch vehicles, was mostly in the form of laths, although~20% considered even as a replacement for the usual maraging plate martensite was also observed. Until 150°C steel. tempering, no noticeable loss of tetragonality wasResearch into this new class of steel, however, has been observed, while the unit cell parameter c/a ratio restricted to process optimisation for best combinations of dropped to almost 1 after 300°C tempering. The yield strength and ductility. 5-8 Although original AFNOR interesting observation at 150°C tempering was the 15CDV6 is reported to be a bainitic grade, 7 little is known predominant presence of fine rodlike e carbide, of the structure/microstructure of the ESR modi ed grade. which may also explain the increased yieldThe aim of the present study was to characterise the role strength. Tempering above 150°C converted the e of hardening and tempering treatments on the development carbide to cementite, relatively thicker precipitates of the microstructure and mechanical properties of this new of similar morphology. At higher tempering ultrahigh strength medium carbon low alloy steel. temperatures, no evidence of spheroidisation of cementites was noted. The highest tempering temperatures of 500 and 600°C resulted in two marked changes in the microstructure: the EXPERIMENTAL METHODS appearance of M 23 C 6 type (Cr, Fe and Mo bearing) Material and processing carbides, and the appearance of, in some regions ofThe ESR modi ed grade, as used in the present study, was the m...

Study of fracture properties of 0·3C–CrMoV(ESR) ultrahigh strength steel

“…The combined effects of the above in association with fine grain structure contribute greatly to enhanced resistance of the steel tempered at 473 K. At higher tempering temperatures, in 0?3C-CrMoV (ESR) steel, the e-carbides are converted into relatively thick cementite platelets of the same morphology along the lath boundaries as well as within the laths. 20 At and above 723 K, relatively recovered regions of microstructure having nearly parallel lath morphology with equiaxed shape are also observed (Fig. 10).…”

“…The presence of retained austenite was observed in a few cases during transmission electron microscopy studies of the steel in the as hardened condition (Fig. 7) as well as in specimens tempered at up to 573 K. 20 This retained austenite was heavily dislocated and rarely exceeded 100-200 nm in size, and disappeared on tempering above 573 K. Fine rodlike precipitates started precipitating out on tempering at low temperatures up to 423 K, and were identified as mostly hexagonal e-carbide by electron diffraction (Fig. 8).…”

Development of process for thermomechanical treatment of ultrahigh strength steel prepared by electroslag refining

“…Tempering in the range of 100-200uC in fact increases the yield strength due to incipient carbide precipitation. Suresh et al 13 found such an increase in strength on tempering oil quenched samples of 0 . 30CrMoV (ESR) steel.…”

Ultrahigh Strength Steel: Development of Mechanical Properties Through Controlled Cooling

“…Many alloys for critical application now use this process to ensure reliability and good properties. The material developed earlier at Indian Institute of Technology (IIT) Bombay and Vikram Saravai Space Center (VSSC), Trivandrum, India with a yield strength of 1450 MPa, is qualified as aerospace application (Suresh et al, 2003;Chatterjee et al, 1990). This was a medium-carbon low alloy steel used mostly in tempered condition.…”

“…This was a medium-carbon low alloy steel used mostly in tempered condition. The chemical composition of the alloy is: 0.3% C, 1.0% Mn, 1.0% Mo, 1.5% Cr, 0.3% V and named as 0.3C-CrMoV (ESR) steel (Suresh et al, 2003). The microstructure of heat treated alloy primarily consists of tempered lath martensite.…”