Precipitation-hardened Martensitic Stainless Steel Research Articles

Microstructure, corrosion behavior, and fatigue resistance of laser powder bed fusion-produced precipitation-hardening martensitic M789 stainless steel

Additive manufacturing has facilitated the fabrication of stainless steel components with intricate designs, providing exceptional strength and corrosion resistance. This advancement, however, poses challenges in accurately predicting fatigue resistance under high-cycle loading, especially when surface features resulting from the printing process and corrosion are overlooked. This study investigates the microstructure of laser powder bed fused Böhler M789 AMPO stainless steel aiming to elucidate its relationship with corrosion resistance and fatigue properties.Through a comprehensive examination of the microstructural features using El-Haddad’s model and extreme value extrapolations, this research reveals the predominant influence of surface roughness, resulting from the printing process, on the fatigue strength of net-shaped specimens. For a stress ratio of 0.1, the estimated fatigue strength ranges between 69 MPa and 162 MPa. Upon mechanically polishing the surface to eliminate the surface roughness effects, the significance of other features is highlighted. Particularly, the effect of corrosion pits, shallow pores and oxides, and internal discontinuities follow in importance.Moreover, this study evaluates the impact of the as-received-oxide layer on corrosion resistance. Upon removal of this layer via surface etching or polishing, an improvement in the stability of the re-formed passive layer was observed. This enhancement is evident in the expanded potential range between the pitting and corrosion potentials, increasing from 0.2 V for the net-shaped surface to 1.05 V for the polished one.This work emphasizes the significance of characterizing microstructural characteristics to predict both fatigue and corrosion resistances. It also highlights potential research areas in additively-manufactured stainless steels.

Data Mining Applied to the Electrochemical Noise Technique in the Time/Frequency Domain for Stress Corrosion Cracking Recognition

In this paper, time/frequency domain data processing was proposed to analyse the EN signal recorded during stress corrosion cracking on precipitation-hardening martensitic stainless steel in a chloride environment. Continuous Wavelet Transform, albeit with some limitations, showed a suitable support in the discriminatory capacity among transient signals related to the different stress corrosion cracking mechanisms. In particular, the aim is to propose the analysis of electrochemical noise signals under stress corrosion cracking conditions in the time–frequency domain by using the Hilbert–Huang approach. The Hilbert–Huang Transform (performed by the Empirical Mode Decomposition approach) was finally proposed to carry out an identification of the corrosion mechanisms in comparison to conventional data processing methods. By using this approach, a detailed simultaneous decomposition of the original electrochemical noise data in the time and frequency domain was carried out. The method gave useful information about transitions among different corrosion mechanisms, allowing us to (i) identify a specific characteristic response for each corrosion damaging phenomenon induced by stress corrosion cracking, (ii) time each corrosion of the damaging phenomenon, and (iii) provide a topological description of the advancing SCC damaging stages. This characteristic evidences that the Hilbert–Huang Transform is a very powerful technique to potentially recognize and distinguish the different corrosion mechanisms occurring during stress corrosion cracking.

Additive manufacturing multi-material components of SAF 2507 duplex steel and 15-5 PH martensitic stainless steel

The study reports the successful deposition of 15–5 precipitation-hardened (PH) martensitic steel on SAF 2507 duplex stainless steel (and vice versa) to form functionally graded materials (FGMs) using powder-based, directed-energy-deposition (DED) technology. The evolution of the microstructure and the mechanical properties of the 15–5 PH/SAF 2507 functionally graded material in the as-built state were investigated systematically. The results proved that the microstructural transition zone (MTZ) is formed above the fusion line due to the dilution effect of both steels. The phase composition of the MTZ consists of ferrite and, in comparison to the base materials, an increased amount of austenite. The interface has the lowest hardness owing to the formation of a significant proportion of the austenitic phase. However, the tensile mechanical properties were not affected by the interface as failure occurred in both SAF 2507 and at the interface regions. The research presents a promising application of FGMs in a horizontal configuration to form a high - quality, metallurgical joint between heterogeneous materials. This study introduces a novel approach by additive manufacturing (AM) of heterogeneous multi-materials, merging the favorable properties of duplex SAF 2507 and martensitic 15-5 PH stainless steels through superior metallurgical bonding. The combination of SAF 2507 and 15-5 PH in a functionally graded material has not been previously explored, as the existing studies deal with the duplex stainless steels SAF 2507 and either martensitic stainless-steel 15-5 PH deposited separately by laser powder-bed-fusion (L-PBF) or DED methods. This research pioneers the investigation of these materials in tandem, paving the way for the development of novel FGMs with optimized properties for specific applications. The resulting FGM exhibits exceptional corrosion resistance and high strength, making it highly versatile for applications in diverse industries such as offshore oil and gas production, aerospace, aviation, and chemical processing equipment.

Effect of heat input on the microstructure and mechanical properties of the deposition obtained by selective laser melting (SLM) of precipitation hardened martensitic stainless steel

The additive manufacturing (AM) shows a great market prospect in the closed-impeller for the centrifugal compressor. In the paper, the precipitation hardened martensitic stainless steel (PHMSS) deposition with different heat input parameters by the selective laser melting (SLM) and the micro-Vickers hardness, tensile strength and impact toughness were detected respectively. The evolution laws of microstructure were analyzed by the SEM, TEM, EDS and EBSD. The tensile strength of the deposition with heat input of the 4.21 J/cm were best and the strength along the build surface direction (X and Y) and the build height direction (Z) were 1193.3 MPa、1222.1 MPa、1159.8 MPa respectively. The impact toughness of the deposition were lower than that of base metas and the tonghness between the sureface direction and the bulid heght direction was anisotropy. The crystals showed a conlumanr pattern along the Z direction and with the increase the heat input, the preferred orientation along the Z changed from the preferred crystal plane and crystal orientation to the pure preferred crystal orientation. A large amount of Cr3C2 phase is precipitated and its size increased with the increase of heat input. The above research results normalize the influence of each parameter during the AM on the microstructure and mechanical properties of the deposition to the heat input, which is helpful to simplify the optimization of the procedure specification and promote its industrial application.

Effect of austenite fraction and stability on strength-hardening-ductility in additively manufactured 17-4 PH stainless steel containing nitrogen

Additively-manufactured (AM) 17-4 precipitation-hardening (PH) martensitic stainless steel (SS) built from nitrogen-atomized powder often retains a large volume fraction of austenite. The retained austenite lowers the yield strength compared to both wrought and AM 17-4 SS built from argon-atomized powder, hindering its use in many applications. However, retained austenite allows for high work hardening and high ductility through deformation-induced martensitic transformation (DIMT). This work systematically investigates the influence of various heat treatments on the volume fraction and stability of the retained austenite from tensile loading in AM 17-4 SS containing a mass fraction of 0.13% nitrogen (AM 17-4N SS). Synchrotron-based high-energy x-ray diffraction (HEXRD) is used to quantify the austenite volume fraction resulting from the heat treatments and the extent of DIMT in tensile specimens loaded to various plastic strains and specimens loaded to fracture. We show that by altering the volume fraction and stability of the retained austenite in AM 17-4N SS using different heat treatments, the DIMT process can be utilized to vary the strength, work hardening, and ductility, thus enabling its use in a wide range of applications. The AM 17-4N SS is also shown to meet or exceed the peak aged requirements for wrought 17-4 SS with yield strength (YS) of >1200 MPa, an ultimate tensile strength (UTS) of >1500 MPa, and a uniform elongation of ≥13% while retaining ≈ 20% volume fraction of austenite.

Optimisation of precipitation hardening for 15-5 PH martensitic stainless steel produced by wire arc directed energy deposition

This paper discusses directed energy deposition of 15-5 PH, which was successfully tailored with precipitation hardening (PH) to achieve the desired properties. Parametric analysis of solution annealing and aging at various time and temperature combinations was performed. Material characterisation was done in the as-deposited, solution annealed, and PH states. Investigations included microscopy, SEM/EDS, fractography, hardness, tensile, and impact toughness test. The as-deposited microstructure was composed of martensite laths along with delta ferrite. Optimisation of solution annealing was mandatory to achieve homogeneous austenite, which allowed PH. PH resulted in similar properties compared to conventionally produced steel. Peak aging resulted in 450 HV10 and an Rm of 1350 MPa, while the over-aged condition resulted in an impact toughness of over 77 J/cm2.

Simultaneous enhancement of strength-ductility via multiple precipitates and austenite in a novel precipitation-hardened martensitic stainless steel

In this study, the advantages of the novel precipitation-hardened martensitic stainless steel (abbreviated as novel steel) in terms of an excellent strength–ductility balance are fully elucidated utilizing various methods and tests. The novel steel increases the yield and ultimate tensile strength from 1480 ± 20 MPa and 1500 ± 18 MPa to 1530 ± 15 MPa and 1650 ± 20 MPa, respectively, and simultaneously improves the elongation from 9.7 ± 0.5% to 17.8 ± 1.1% over that of conventional PH13–8Mo steel. Specifically, the novel steel is mainly strengthened and toughened by three factors, including fine grain strengthening, precipitation strengthening of multiple precipitates (Cu, NiAl) and the TRIP effect. The contribution of fine grain strengthening is quantitatively evaluated through the Hall–Petch relationship. Meanwhile, the value of strengthening contributions by multiple precipitates are quantitatively evaluated through the shearing or Orowan mechanism. Significantly, compared with conventional PH13–8Mo steel, the addition of Cu leads to the formation of multitudinous metastable austenite in the novel steel. The TRIP effect can be generated by metastable austenite upon applied loading conditions. An accessible pathway is found to exploit high-performance stainless steel.

Effect of Hot Torsion on Microstructure and Properties of Precipitation Hardening Martensitic Stainless Steel

Effect of Hot Torsion on Microstructure and Properties of Precipitation Hardening Martensitic Stainless Steel

A novel heat treatment for improving the hydrogen embrittlement resistance of a precipitation-hardened martensitic stainless steel

Application of the novel heat treatment significantly improves the PH13–8Mo steel hydrogen embrittlement (HE) resistance over that obtained by conventional heat treatment, while maintaining the mechanical properties. The mechanism of the proposed method is as follows: austenite captures hydrogen and the mechanical properties are improved by fine grain strengthening. The results of this study show that compared with the steel properties obtained by conventional peak-aged heat treatment, the novel heat treatment results in a reduction in the tensile strength from 1500 MPa to 1400 MPa, but a significant decrease in the HE susceptibility index (IHE) from 52% to 14%.

Structure and Properties of Gas-Nitrided, Precipitation-Hardened Martensitic Stainless Steel.

Nanoflex stainless steel is a promising material for medical applications. However, improvement of its mechanical properties without compromising its corrosion resistance is still a challenge. In order to investigate the effect of the nitriding process on the corrosion and wear resistance of Sandvik NanoflexTM steel, a number of processes were carried out in a gas atmosphere with differing ammonia contents in the temperature range of 425–475 °C for 4 h. The mechanical properties and wear resistance of the layers were tested using the nanoindentation and pin-on-disc methods, respectively. In order to assess corrosion resistance, potentiodynamic tests were carried out in Ringer’s artificial body fluid and in a 3% aqueous solution of sodium chloride. The results are discussed herein with respect to the microstructural characteristics of the layers studied using light and scanning electron microscopy, X-ray diffraction phase analysis and wavelength dispersive X-ray microanalysis. The structure of nitrided layers included three zones: the subsurface zone composed of nitrides and the zones composed of metastable phases, i.e., the S phase (γN) and expanded martensite (αN) with possible precipitates of nitrides. The third zone adjacent to the steel core was enriched with carbon. The nitrided samples showed significant improvement in the wear rate while maintaining good corrosion resistance in comparison to the non-treated steel. We concluded that nitriding should be carried out at a temperature below 450 °C and in an atmosphere containing no more than approximately 50% ammonia in order to avoid nitrides precipitation.

Atomic-scale characterization of multiple precipitating species in a precipitation-hardened martensitic stainless steel

Atomic-scale characterization of multiple precipitating species in a precipitation-hardened martensitic stainless steel

How Austenitic Is a Martensitic Steel Produced by Laser Powder Bed Fusion? A Cautionary Tale

Accurate phase fraction analysis is an essential element of the microstructural characterization of alloys and often serves as a basis to quantify effects such as heat treatment or mechanical deformation. Additive manufacturing (AM) of metals, due to the intrinsic nonequilibrium solidification and spatial variability, creates additional challenges for the proper quantification of phase fraction. Such challenges are exacerbated when the alloy itself is prone to deformation-induced phase transformation. Using commonly available in-house X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) and less commonly used synchrotron-based high-energy X-ray diffraction, we characterized nitrogen-atomized 17-4 precipitation-hardening martensitic stainless steel, a class of AM alloy that has received broad attention within the AM research community. On the same build, our measurements recovered the entire range of reported values on the austenite phase fractions of as-built AM 17-4 in literature, from ≈100% martensite to ≈100% austenite. Aided by Calphad simulation, our experimental findings established that our as-built AM 17-4 is almost fully austenitic and that in-house XRD and EBSD measurements are subject to significant uncertainties created by the specimen’s surface finish. Hence, measurements made using these techniques must be understood in their correct context. Our results carry significant implications, not only to AM 17-4 but also to AM alloys that are susceptible to deformation-induced structure transformation and suggest that characterizations with less accessible but bulk sensitive techniques such as synchrotron-based high energy X-ray diffraction or neutron diffraction may be required for proper understanding of these materials.

Effect of Laser Metal Deposition Parameters on the Characteristics of Stellite 6 Deposited Layers on Precipitation-Hardened Stainless Steel.

Laser metal deposition (LMD) is one of the manufacturing processes in the industries, which is used to enhance the properties of components besides producing and repairing important engineering components. In this study, Stellite 6 was deposited on precipitation-hardened martensitic stainless steel (17-4 PH) by using the LMD process, which employed a pulsed Nd:YAG laser. To realize a favor deposited sample, the effects of three LMD parameters (focal length, scanning speed, and frequency) were investigated, as well as microstructure studies and the results of a microhardness test. Some cracks were observed in the deposited layers with a low scanning speed, which were eliminated by an augment of the scanning speed. Furthermore, some defects were found in the deposited layers with a high scanning speed and a low frequency, which can be related to the insufficient laser energy density and a low overlapping factor. Moreover, various morphologies were observed within the microstructure of the samples, which can be attributed to the differences in the stability criterion and cooling rate across the layer. In the long run, a defect-free sample (S-120-5.5-25) possessing suitable geometrical attributes (wetting angle of 57° and dilution of 25.1%) and a better microhardness property at the surface (≈335 Hv) has been introduced as a desirable LMDed sample.

Correlation between the microstructure and hydrogen embrittlement resistance in a precipitation-hardened martensitic stainless steel

Custom 465 precipitation-hardened stainless steel was subjected to various thermal treatments and then evaluated using tensile testing with a slow strain rate after electrochemical hydrogen charging. The investigation revealed that cryogenically-treated and thermally-aged (593 ℃) specimens exhibited superior resistance to hydrogen embrittlement. Prolonging the aging time reduced the hydrogen susceptibility, which was primarily attributed to the increased stability of reversed austenite caused by the dissolution and transformation of η-Ni3Ti. Stress concentration relaxation due to the absence or ripening of η-Ni3Ti further reduced the hydrogen susceptibility. Additionally, hydrogen-induced subcritical cracks initiated on the sample surface governed the fracture behavior.

Hydrogen Uptake, Tensile, and Fatigue Properties of a Barrier-Coated, Precipitation-Hardened Martensitic Stainless Steel With Exposure to High-Pressure Hydrogen Gas

Abstract Hydrogen uptake, tensile, and fatigue properties of a precipitation-hardened martensitic stainless steel with a newly developed coating (alumina/aluminum/Fe–Al) were presented. The developed coating had an excellent resistance to hydrogen entry in 100-MPa hydrogen gas at 270 °C. Measurements of bulk and local hydrogen by thermal desorption analysis and secondary-ion mass spectrometry (SIMS) suggested that the excellent resistance was attributed to the reduction in permeation areas by interfacial hydrogen trapping between the aluminum and Fe–Al layers. Tensile tests of a smooth, round-bar specimen, and fatigue tests of a circumferentially notched specimen after exposure to 100-MPa hydrogen gas at 270 °C were performed in air at room temperature (RT). These properties of the coated specimens were not degraded by hydrogen exposure, whereas those of the noncoated specimens were significantly degraded. Hydrogen-pressure cycle tests of coated, tubular specimens with an inner notch in 95-MPa hydrogen gas at 85 °C also demonstrated that the fatigue life was improved by the coating.

Formability, Microstructure and Mechanical Properties of Flow-Formed 17-4 PH Stainless Steel

The subject of the research that has been conducted in this paper was to analyze precipitation-hardened martensitic stainless steel 17-4 PH after flow forming with four different strains and subsequent standard heat treatment. Four cylinders were obtained with a 16, 30, 48 and 68% reduction in thickness, respectively. The samples were analyzed in terms of their mechanical properties and microstructural changes before and after the heat treatment. The results showed that a higher strain resulted in an overall higher strength (up to 1200 MPa UTS) and refinement of the structure, although at a cost of the elongation. High deformation influenced the precipitation process, and the ratio of the grain boundaries significantly increased. Nonetheless, comparing the obtained results with other similar research, it seems that the formation of nano-precipitates of Cu is the key-strengthening mechanism. Strain hardening contributes to an increase in the strength of the steel, although the effect decreases after heat treatment. The relatively small values of residual stress in the steel, especially after the heat treatment, confirmed these claims. Overall, flow forming allowed high deformations of the 17-4 PH steel to be obtained although it did not significantly change the mechanical properties of the material due to the dominant precipitation hardening mechanism.

Quantitative electron microscopy and physically based modelling of Cu precipitation in precipitation-hardening martensitic stainless steel 15-5 PH

Precipitation-hardening martensitic stainless steels rely on very fine precipitates for optimal mechanical performance. These multicomponent alloys are prone to clustering and precipitation reactions during tempering, where Cu is one of the alloying elements added to stimulate precipitation. It is efficient to use an integrated computational materials engineering (ICME) approach to tailor alloying and heat treatment for design of these alloys. The most promising physically based modelling of precipitation for this purpose at present is Langer-Schwartz-Kampmann-Wagner (LSKW) modelling within the CALPHAD framework. This approach has been successful for model alloys, but reliable results for multicomponent stainless steels are less common. Hence, we combine quantitative transmission electron microscopy and LSKW modelling to investigate the tempering of a martensitic stainless steel 15-5 PH at 500°C. The microstructural characterization shows that the Cu precipitation and growth occur in three stages: i) Cu BCC, ii) Cu 9R, and iii) Cu FCC, during tempering up to 1000h. The modelling predictions of size, volume fraction and number density of precipitates are in good agreement with the experimental results. Thus, the approach with a combination of quantitative electron microscopy and LSKW modelling using CALPHAD-type databases holds promise for further optimization of precipitation-hardening martensitic stainless steels.

The interaction of corrosion fatigue and stress-corrosion cracking in a precipitation-hardened martensitic stainless steel

Fracture mechanics-based testing was used to quantify the stress-corrosion cracking and corrosion fatigue behavior of a precipitation-hardened martensitic stainless steel (Custom 465-H950) in full immersion chloride-containing environments at two applied electrochemical potentials. A plateau in the cycle-based crack-growth kinetics (da/dN) was observed during fatigue loading at low ΔK and [Cl−] at and above 0.6 M. Evaluation of the fracture morphology and frequency dependence of this plateau behavior revealed an intergranular fracture surface morphology and constant time-dependent growth rates. These data strongly support a controlling stress-corrosion cracking mechanism occurring well below the established KISCC for quasi-static loading. Low-amplitude cyclic loading below ΔKTH (i.e., “ripple loads”) is hypothesized to enable time-dependent intergranular-stress-corrosion cracking to occur below the KISCC via mechanical rupturing of the crack-tip film and enhancement of the H embrittlement-based SCC mechanism.

Laser-based welding of 17-4 PH martensitic stainless steel in a tubular butt joint configuration with a built-in backing bar

Laser-based welding of thick 17-4 precipitation hardening (PH) martensitic stainless steel (SS) plates in a tubular butt joint configuration with a built-in backing bar is very challenging because the porosity and cracks are easily generated in the welds. The backing bar blocked the keyhole opening at the bottom surface through which the entrapped gas could escape, and the keyhole was unstable and collapsed overtime in a deep partially penetrated welding conditions resulting in the formation of pores easily. Moreover, the fast cooling rate prompted the ferrite transform to austenite which induced cracking. Two-pass welding procedure was developed to join 17-4 PH martensitic SS. The laser welding assisted by a filler wire, as the first pass, was used to weld the groove shoulder. The added filler wire could absorb a part of the laser beam energy; resulting in the decreased weld depth-to-width ratio and relieved intensive restraint at the weld root. A hybrid laser-arc welding or a gas metal arc welding (GMAW) was used to fill the groove as the second pass. Nitrogen was introduced to stabilize the keyhole and mitigate the porosity. Preheating was used to decrease the cooling rate and mitigate the cracking during laser-based welding of 17-4 PH martensitic SS plates.

Microstructure and mechanical properties of hot wire laser clad layers for repairing precipitation hardening martensitic stainless steel

Abstract Precipitation hardening martensitic stainless steel (PH-MSS) is widely used as load-bearing parts because of its excellent overall properties. It is economical and flexible to repair the failure parts instead of changing new ones. However, it is difficult to keep properties of repaired part as good as those of the substrate. With preheating wire by resistance heat, hot wire laser cladding owns both merits of low heat input and high deposition efficiency, thus is regarded as an advantaged repairing technology for damaged parts of high value. Multi-pass layers were cladded on the surface of FV520B by hot wire laser cladding. The microstructure and mechanical properties were compared and analyzed for the substrate and the clad layer. For the as-cladded layer, microstructure was found non-uniform and divided into quenched and tempered regions. Tensile strength was almost equivalent to that of the substrate, while ductility and impact toughness deteriorated much. With using laser scanning layer by layer during laser cladding, microstructure of the clad layers was tempered to fine martensite uniformly. The ductility and toughness of the clad layer were improved to be equivalent to those of the substrate, while the tensile strength was a little lower than that of the substrate. By adding TiC nanoparticles as well as laser scanning, the precipitation strengthening effect was improved and the structure was refined in the clad layer. The strength, ductility and toughness were all improved further. Finally, high quality clad layers were obtained with equivalent or even superior mechanical properties to the substrate, offering a valuable technique to repair PH-MSS.