A Comparison of Mechanical Properties and Hydrogen Embrittlement Resistance of Austempered vs Quenched and Tempered 4340 Steel

Abstract

This study was conducted to compare the hydrogen embrittlement (HE) resistance of austempered 4340 steel with quenched and tempered (Q&T) 4340 steel with an identical yield strength (YS) of 1340 MPa (194 ksi). A baseline comparison showed that the austempered steel with a lower bainite microstructure exhibited higher hardness, tensile strengths, Charpy V-notch (CVN) impact toughness, and ductility at both low 233 K (-40 F) and ambient temperatures, as compared to the Q&T steel with a martensite microstructure. After machining and just prior to testing, subsized CVN specimens and notched bend specimens were immersed in hydrochloric acid-water baths. The HE resistance was higher for the austempered steel than the Q&T steel. No differences in room-temperature CVN energy resulted from hydrogen charging of the austempered and Q&T steels vs their unexposed counterparts. However, in the notched bend specimens, the hydrogen charging caused significant peak load decreases (40 pct) for the Q&T steel, while the austempered steel exhibited only small (6 pct) decreases in peak load. Intergranular (IG) fracture occurred solely in the charged Q&T bend samples, which is further evidence of their embrittlement.

Comparison of mechanical properties of martensite/ferrite and bainite/ferrite dual phase 4340 steels

“…For comparison the hardness and charpy impact energy of full martensite and bainite steels are also measured. It can be seen from Table 3 that bainitic steel has greater impact energy and elongation than martensitic steel which is in agreement with the results of Tartaglia et al [6]. They reported that lower bainite is tougher than tempered martensite in AISI 4340 steel.…”

Effect of the Austempering Process on the Microstructure and Mechanical Properties of 27MnCrB5-2 Steel

“…Their strength and impact toughness could be further improved by replacing conventional Q&T with austempering, which leads to bainitic or mixed bainite-martensite microstructures [1,2].…”

The Effects of Martensite Content on the Mechanical Properties of Quenched and Tempered 0.2%C-Ni-Cr-Mo Steels

“…After tempering to the same hardness, the ductile-to-brittle impact transition temperature for any carbon level was 25-50°C (45-90°F) lower for fully martensitic microstructures than for microstructures containing only 50% martensite. This author and others have shown that SAE 4340 steel has better impact toughness when it is austempered (isothermally transformed) to 100% lower bainite than when it is quenched to 100% martensite and tempered to the same strength level ( Ref 1,2). However, steels with mixed microstructures resulting from incomplete bainitic treatments and partial transformation to martensite possess much lower toughness (higher transition temperatures) than steels with microstructures containing either 100% lower bainite or 100% tempered martensite ( Ref 1).…”