Microstructural evolution in a PH13-8 stainless steel after ageing
Introduction
PH13-8 stainless steel is a martensitic precipitation hardening (PH) steel. It has high strength and hardness with good levels of resistance to both general corrosion and stress-corrosion cracking. In addition, the alloy exhibits good ductility and toughness in large sections in both the longitudinal and transverse directions, and offers a high level of useful mechanical properties under severe environmental conditions superior to PH17-4 and PH15-5 stainless steels. It has been used for many applications, such as landing gear parts, nuclear reactor components and petrochemical applications requiring resistance to stress-corrosion cracking [1].
A number of studies have shown that the ferritic and martensitic phases in high alloy steels (including stainless steels such as PH13-8 steel) containing nickel and aluminium can be hardened by ageing at temperatures above 400 °C [2], [3], [4], [5]. The strengthening is reported to be due to the precipitation of the ordered phase NiAl which has a B2 (CsCl) superlattice structure. However, due to the instrument limitation, in the past, it was not possible to analyse directly fine precipitates of nanometer scale. Therefore, the materials studied had to be treated to seriously-overaged conditions so as to achieve precipitates of detectable sizes by transmission electron microscope (TEM) [6], [7]. The most recent study with wrought PH13-8 stainless steel was carried out by Seetharaman et al. in early 1980s [6]. They found that precipitates formed during ageing at temperatures lower than 500 °C for 4 h could not be resolved with TEM. When samples overaged at 575 °C were studied, the precipitates were claimed to be spherical, and uniformly distributed in the matrix. The selected area diffraction pattern (SADP) analysis revealed the existence of the intermetallic compound NiAl of B2 structure. Another relevant study was carried out by Taillard et al. on Fe-19wt%Cr-Ni-Al systems [8], [9]. They found that the NiAl precipitates were homogeneously nucleated and coherent with the matrix after 400 min at 650 °C. It should be noted that the precipitation information obtained in seriously-overaged conditions does not necessarily represent what happens in the commercially treated materials, where the ageing treatment is usually 1 or 4 h at temperatures below 600 °C. The most recent work on PH13-8 stainless steel is on a cast grade [7]. The precipitate type was reported as NiAl through SADP analysis and observed to be spherical (H1150M treatment, see Section 2.1 for details). It was very difficult to image the precipitates using TEM even after ageing at 510 °C for 4 h. The size only reaches 40–50 nm after the H1150M treatment. It is worthwhile mentioning that the age hardening kinetics of the cast grade differs from its wrought counterpart significantly [10]. Other previous work was also carried out on overaged materials, as otherwise the precipitates were not large enough to allow TEM examination and SADP identification [11], [12]. How and when the precipitates start to form during ageing of the PH13-8 alloy, and how they evolve in terms of size, composition and even type remain unclear.
Atom probe field ion microscopy (APFIM) proved to be very powerful in the investigation of small precipitates when other microscopy techniques are unsatisfactory [13], [14]. Its unique capability of measuring composition variations in a nanometer scale, together with equal detection efficiency for all elements, make it particularly suitable for investigation of early stages of precipitation. The early atom probe (AP) analysis was essentially one dimensional in nature. Recent developments allow a 3-dimensional reconstruction of the distribution of different atoms in the analysed volume [15]. Atom probe (AP) microanalysis was carried out in determining the composition of the precipitate in an Fe-20Cr-2Ni-2Al (at%) alloy [16]. It was found that the composition of the B2 intermetallic phase is close to the stoichiometric composition NiAl with only a limited amount of dissolved iron, which decreases with ageing time. The treatments used in their study were ageing at 550 °C for 6, 17 and 117 h. Miller and Hetherington studied an Fe-Ni-Al-Mo system [17]. They found that Mo partitioned preferentially to the α matrix, and NiAl β’ precipitates also contained approximately 11% Fe.
Some previous work with Cr-containing steels is summarised here since the studied PH13-8 steel contains chromium. Most of the alloys previously studied contain a higher amount of Cr than PH13-8. Alloys Fe-30.1Cr-9.9Co, Fe-20.2Cr-8.8Al-0.55Ti, and Fe-26Cr-(0, 3, 5, or 8)Ni (all in at%) all show spinodal decomposition of Cr during the ageing process [18], [19], [20]. Stainless steel PH17-4 (0.3C-17.5Cr-4Ni-3Cu-lSi in at%) was found to be strengthened by ϵ-Cu particles produced during ageing [21]. Murayama et al. [22] found that the phase separation occurs through a spinodal mechanism while ageing at 400 °C. Tempering this alloy at 580 °C for 4 h does not lead to any Cr separation in the martensite phase, which is the condition before this alloy enters service. Another maraging stainless grade 1RK91 (13Cr-9Ni-2Mo-2Cu in wt%) is of low-Cr type. No α-α’ transformation, i.e. the spinodal decomposition of Cr, was found in this alloy at 475 °C even after 1000 h [23]. The good mechanical properties are attributed to the heterogeneous precipitation of Ni-rich/(Al-Ti) precipitates, possibly Ni3(Ti,Al) type, on the Cu particles formed in the very early stages of ageing [24]. Enrichment of Mo at the precipitate/matrix interface impedes the coarsening of particles during ageing. An aged ferritic-martensitic steel (with 11 at% Cr) was investigated using atom probe [25]. After ageing at 400 °C for 17000 h, long range fluctuations with composition difference of 2.9 at% were present in the alloy.
From the above summary of the previous work, it is clear that there is lack of information in a few aspects about PH13-8 steel during ageing, such as, when and how the precipitates start to form during ageing; how they evolve in terms of type, composition, shape, size and fraction, and what kind of precipitates contribute to the strengthening effects. It will also be useful to determine whether Cr decomposes spinodally in a steel of such a low Cr level. If so, how fast does this take place? What will be the influence on the evolution of other precipitates and strengthening kinetics? Does Mo segregate to the precipitate/matrix interface, as reported for the 1RK91 steel? Answers to the above questions will help in understanding the strengthening mechanisms during ageing, and make possible a computer model of the age hardening kinetics based on existing precipitate hardening theories [26], [27].
Section snippets
Material
The typical ageing treatment for PH13-8 steel is ageing the solution-treated material at intermediate temperatures, for instance, 510 °C for 4 h (H950), 593 °C for 4 h (H1100), or 760 °C for 2 h followed by 4 h at 621 °C (H1150M). Such treatments cause hardening by precipitation of intermetallic compounds and provide a good combination of strength and ductility [1].
The material studied was provided by Allvac Ltd., UK. Its composition in both wt% and at% is given in Table 1. The material was
Result analysis
The hardness measurement was carried out on a Mitutoyo HM-124 machine, with 2 kg working load. The age hardening curves at 510 and 593 °C of the studied material are plotted in Fig. 1. The hardness reaches peak after 30 min at 593 °C, but still increases after ageing for 4 h at 510 °C. The ageing behaviour is similar to what was obtained before for a wrought PH13-8 stainless steel [1], but significantly different from that of a cast PH13-8 steel [7], [10]. The PoSAP machine was first operated
Microstructure evolution during ageing
The amount of retained austenite formed after solution treatment was studied before [7]. The relative amounts of austenite (in wt%) in PH13-8 stainless steel were determined by Mossbauer spectroscopy. After homogenisation at 1121 °C for 4 h (furnace-cooled), then solution treatment at 927 °C for 1.5 h (fan-cooled), with no refrigeration treatment, about 0.4±0.5% retained austenite was detected. Seetharaman et al. solution-treated PH13-8 at 900 °C for 30 min and their x-ray diffraction results
Conclusions
The current study of the precipitation process during ageing in a wrought PH13-8 steel has led to the following conclusions.
- 1.
Detectable precipitates form after 40 min ageing at 510 °C, or 6 min at 593 °C. They are enriched in Ni and Al, but depleted of Fe and Cr. The amount of Ni and Al increases when ageing proceeds, but the precipitate composition is far from the stoichiometric NiAl B2 phase after 4 h at 510 °C (H950), or 30 min at 593 °C.
- 2.
Ageing for 40 min at 510 °C results in spherical
Acknowledgements
G.D.W. Smith and A. Cerezo in the Department of Materials at Oxford University are thanked for providing the position sensitive atom probe facility and software for data analysis, and useful discussions. Thanks are also given to T.J. Godfrey and other personnel working in the atom probe laboratory. This work was carried out within the project of ‘Computer Modelling of the Evolution of Microstructure during Processing of Maraging Steels’ sponsored by the Engineering and Physical Sciences
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