Improving Toughness of PH13-8 Stainless Steel through Intercritical Annealing

Abstract

An intercritical annealing step was introduced in the treatment of a PH13-8 stainless steel to improve the toughness of the alloy in aged condition. Four different treatment cycles, i.e. austenitisation (Q-treatment) and intercritical anneal (L-treatment), LQ, 2B (QLQL) and 2K (LQLQ) were carried out before aging treatment at 510°C for 4 h (the commercial H950 treatment). Optical and scanning electron microscopies, and Xray diffraction analysis were employed to study the microstructures of the alloys after different heat treatments. Hardness and Charpy impact strength of the samples were measured.Results show that significant grain refinement was observed after 2K and 2B treatment, but not after QL and LQ treatments. Such refinement of prior austenite grain did not lead to significant increase of hardness either before or after ageing. The Charpy impact strength of the alloy in aged condition was improved after the four pre-ageing treatments. The formation of a 'dual-phase' martensitic structure through intercritical annealing is thought to make the main contribution to the better toughness obtained, with beneficial effects also from grain refinement. All the four treatments offer better combined properties than the commercial treatment, whereas QL and LQ treatments may be cost-competitive. Relationships among heat treatment, microstructure and mechanical properties are discussed.KEY WORDS: stainless steel; toughness; grain refinement; intercritical annealing; heat treatment.Such grain refinement increased the strength of the alloy at room temperature, and a significant increase in ultimate tensile strength was observed at elevated temperatures. Recently Guo et al. reported that introducing an intercritical annealing step before the solution treatment of AerMet 100 (Fe-13.4Co-11.1Ni-3.1Cr-1.2Mo-0.23C in wt%), a high strength lath martensitic steel, may result in fine effective grain size.11) It should be noted that when maraging steels, i.e., martensitic precipitation hardening steels were studied, [7][8][9][10] attempts on grain refinement was through rapid austenitisation procedures, whereas studies on other alloys employed intercritical annealing in their thermal treatments. [2][3][4][5][6]11) In the latter cases, even when tempering treatments were included, 3,4) the purpose was to introduce thermally stable austenite but not to achieve precipitation hardening. The grain size was retained through the tempering step.3,4) It is not yet clear how the introduction of intercritical annealing will affect the precipitation kinetics and mechanical properties of precipitation hardening steels.The aim of the current research was to refine the grain size of PH13-8 stainless steel through the introduction of intercritical annealing treatment. The influence of thermal treatment on grain size, mechanical properties of the alloy before and after ageing, and precipitation kinetics was studied. The relationships among heat treatment, microstructure, and mechanical properties are also discussed. Experimental Procedures Alloy ...

Influence of tempered microstructures on the transformation behaviour of cold deformed and intercritically annealed medium carbon low alloy steel

“…The relatively homogeneous duplex structures observed for the M650 specimens is due to the greater microstructural stability achieved by tempering at 650 °C, as there is less precipitation from the structure during the of the intercritical annealing treatment 14,23 . This same phenomenon was observed for normalized structures which yielded very well defined equi-axed duplex structures with optimized strength and plasticity characteristics, which was attributed to the stable nature of the normalized structures 12,14,19 . Thus for the production of well defined equi-axed duplex structures in medium carbon low alloy steels, M300 and M500 initial microstructures are not recommended.…”

“…Initially, dual phase steels were technically defined as steels with microstructure consisting of 30% martensite with the balance ferrite; but as a result of its amazing combination of mechanical properties, there has also been interest in the development of dual phase steels with between 50 -60% martensite derived by the utilization of steels of medium carbon composition. Dual phase steels with 50 -60% martensite have been touted to have the potentials for good combination of high strength, toughness, ductility and fatigue strength -extending the use of dual phase steels for more structural applications 6,[11][12] . The steel chemistry and processing parameters such as intercritical temperature, soaking time, cooling rate, and deformation processes; have been observed to influence the proportion, dispersion and morphology of the dual phase microstructures 2,[13][14][15][16][17] .…”

Hot Deformation and Dynamic Recrystallization of 17-4 PH Stainless Steel

“…Also, it should be indicated that the reported works on grain refining of PH stainless steels are scant. 9,10) …”

Low Nickel Maraging Steel

“…Eventually, the fine-grained microstructure is achieved. In the same paper by Guo et al (2003), it was reported that LQ treatment nearly has no effect on grain refinement, but significant refinement can be obtained if iterating these heating procedures, namely LQLQ treatment (for QL treatment, it should be QLQL treatment).…”

“…7.10a. Guo et al (2003) gave an explanation as to why the two-step martensitic transformation of dual-phase results in the refinement of grain size. Owing to substitutional alloying elements such as Ni, Mo and Al replacing a portion of the position of iron atoms in fcc structure, there are two kinds of austenite formed, namely low-alloy and high-alloy phases, leading to two different quenching responses.…”