Hardness Of Stainless Steel Research Articles

Effect of low-temperature plasma nitriding on the sliding wear behavior of 17–4PH stainless steel in high temperature water

Plasma nitriding significantly enhances the tribological performance of 17–4 precipitation hardening (PH) stainless steel. However, the wear resistance of plasma-nitrided 17–4PH stainless steel in high temperature water remains unpredictable due to the complex interaction between wear and corrosion under such conditions. In this study, we conducted a wear test on nitrided 17–4PH stainless steel in high temperature water to evaluate its wear behavior. Our findings demonstrate that nitriding treatment improves wear resistance. Additionally, the wear mechanism shifts from delamination wear to oxidational wear after nitriding. The interaction between wear and corrosion was revealed. The subsurface of the worn scar undergoes plastic deformation, which accelerates corrosion. However, the oxide scale formed by corrosion can act as a lubricant to mitigate wear. For non-nitrided specimen with lower hardness, delamination cracks initially form in the subsurface, with the crack surfaces subsequently oxidizing and filled with debris. Material loss results from the peeling of the delamination. In contrast, for nitrided specimens with higher hardness, the subsurface of the worn scar is composed of an oxidation layer that detaches under mechanical wear, leading to material loss.

Interfacial Heterogeneous Microstructure and Subsequent Machinability of 316L/CuSn10 Bimetallic Structure Manufactured by Laser Powder Bed Fusion

Additively manufactured bimetallic structures combine the advantages of dissimilar materials and can achieve localized properties through a customized composition distribution. However, additively manufactured parts may still lack the dimensional accuracy and surface integrity essential for precision mechanical assemblies that the post-machining process can address. Therefore, this study aims to systematically investigate the microstructure and machinability of 316L/CuSn10 bimetallic structures fabricated using laser powder bed fusion. The results show that the fusion zone of the bimetallic structure had refined grains of microscale size owing to the mixture of the primary elements of the bimetals, which resulted in the highest microhardness of 3.4 GPa. The difference in microstructure and microhardness between the single-material and fusion zones also causes significant differences in the cutting response during the ultraprecision process. The 316L stainless steel side exhibited the highest cutting force and more severe material accumulation in the chips. The cutting force drops when cutting through the fusion zone, with an observable fracture in the chips and separation of dissimilar materials on the machined grooves, indicating that the heterogeneous properties of additively manufactured 316L/CuSn10 bimetallic structures pose challenges to the improvement of surface quality. The simulation results also showed that stress accumulation occurred in the tool path through the fusion zone owing to the higher yield strength and hardness of stainless steel, indicating that lower cutting speeds and depths of cut are favorable for reducing cutting force and improving surface quality. This study provides deep insight into the microstructure evolution mechanism and a theoretical basis for improving the surface quality of additively manufactured bimetallic structures using an ultraprecision machining process.

The effect of weld heat input on the microstructure and mechanical properties of wire arc additively manufactured 15-5PH stainless steel

Precipitation hardening (PH) stainless steels, such as 15-5PH, have a high strength combined with excellent corrosion resistance. These properties make them valuable in critical industries such as defence, construction, aerospace, energy and maritime. Recent advancements in additive manufacturing (AM) technology enable the rapid and cost-effective production of components. In the case of 15-5PH components manufactured using wire arc additive manufacturing (WAAM), the as-deposited mechanical properties are not suitable at present for industrial applications. This paper explores the mechanical properties of this process and alloy combination without post weld heat treatment with the aim of eventual adoption in this condition by industry. The impact of weld heat input on the microstructure and mechanical properties of stainless steel 15-5PH produced using WAAM was investigated. The microstructure was examined using hardness testing in addition to optical and electron microscopy. Furthermore, mechanical properties were measured with tensile and impact testing. Investigations were conducted on material produced using weld heat inputs of 0.223 kJ/mm and 0.565 kJ/mm. These results indicate that reducing the weld heat input leads to a minor decrease in strength but an 80% increase in impact toughness. This reduction in weld heat input is correlated with a 50% reduction in volume fraction of δ-ferrite while also noting a 55% increase in carbide precipitates. In addition, the fracture surfaces were predominantly cleavage or quasi-cleavage in morphology.

Correlation of microstructure, mechanical properties, and residual stress of 17-4 PH stainless steel fabricated by laser powder bed fusion

17-4 precipitation hardening (PH) stainless steel is a multi-purpose engineering alloy offering an excellent trade-off between strength, toughness, and corrosion properties. It is commonly employed in additive manufacturing via laser powder bed fusion owing to its good weldability. However, there are remaining gaps in the processing-structure-property relationships for AM 17-4 PH that need to be addressed. For instance, discrepancies in literature regarding the as-built microstructure, subsequent development of the matrix phase upon heat treatment, as well as the as-built residual stress should be addressed to enable reproducible printing of 17-4 builds with superior properties. As such, this work applies a comprehensive characterisation and testing approach to 17-4 PH builds fabricated with different processing parameters, both in the as-built state and after standard heat treatments. Tensile properties in as-built samples both along and normal to the build direction were benchmarked against standard wrought samples in the solution annealed and quenched condition (CA). When testing along the build direction, higher ductility was observed for samples produced with a higher laser power (energy density) due to the promotion of interlayer cohesion and, hence, reduction of interlayer defects. Following the CA heat treatment, the austenite volume fraction increased to ∼35 %, resulting in a lower yield stress and greater work hardening capacity than the as-built specimens due to the transformation induced plasticity effect. Neutron diffraction revealed a slight reduction in the magnitude of residual stress with laser power. A concentric scanning strategy led to a higher magnitude of residual stress than a bidirectional raster pattern.

Effect of compositional variations on the heat treatment response in 17-4 PH stainless steel fabricated by laser powder bed fusion

17–4 precipitate hardening (PH) stainless steel is used in various applications including in the aerospace, marine, and chemical industries, largely due to its unique combination of corrosion resistance and high strength, which is achieved by the formation of nanoscale Cu-rich precipitates during aging. 17–4 PH has been widely researched for its applicability for laser powder bed fusion (LPBF). However, there are discrepancies in the literature on its heat treatment response, which seem to be linked to compositional variations. Systematic studies of the interplay between these variations and nanoscale precipitation are currently missing. Using atom probe tomography, we present a systematic study of the heat treatment responses of two variants of LPBF 17–4 PH builds fabricated from different powder feedstocks, with significant differences in N contents (High vs Low N 17–4). Both variants formed predominantly δ-ferritic as-built microstructures. The as-built High N 17–4 variant showed a higher volume fraction of austenite which further increased upon solution annealing and quenching. The consequence was no appreciable hardening effect due to the absence of Cu precipitation in either austenite or martensite after aging, degrading the alloy's desirable property profile. Conversely, Low N 17–4 showed no austenite in the as-built condition and a fully martensitic matrix after solution annealing. This variant had the desired aging response; a ∼ 140 HV 5 increase in hardness due to nanoscale Cu precipitation. Our findings describe the deleterious effects of compositional variations incurred during the LPBF process flow and how they can be overcome in 17–4 PH and similar steels.

Effect of Heat Treatment on Gradient Microstructure and Tensile Property of Laser Powder Bed Fusion Fabricated 15-5 Precipitation Hardening Stainless Steel

Effect of Heat Treatment on Gradient Microstructure and Tensile Property of Laser Powder Bed Fusion Fabricated 15-5 Precipitation Hardening Stainless Steel

Realization of Soft Magnetic Properties in Precipitation Hardening Stainless Steel

Precipitation hardening of soft magnetic stainless steel was investigated to achieve both workability (low hardness during machining) and durability (high hardness in practical use) while maintaining soft magnetic properties. NiAl intermetallic compound was used for precipitation hardening. The relationship among aging temperature, microstructure, magnetic properties, and hardness was investigated for 22 composition samples with the basic composition of 15Cr-1Si-2Mo-3Ni-1Al-0.15Ti-bal. Fe (in wt.%) and varying compositions of Si, Mo, Ni, Al, and Cu. The maximum aged hardness was obtained by solution treatment at 1030 °C followed by aging at 550 °C. Soft magnetic properties were achieved with a single-phase ferrite composition (δ: bcc structure), and the hardness level could be adjusted with the concentration of solid solution strengthening elements. Two indexes of the relationship between microstructure and magnetic properties and between solid solution strengthening elements and hardness were obtained. The soft magnetic properties obtained by solution treatment were maintained after aging despite the occurrence of nonmagnetic precipitates of NiAl intermetallic compound. This is because precipitates do not become pinning sites for the domain wall due to their small size and distance between precipitates (1/175 and 1/26 of the domain wall, respectively). As a result, a practical precipitation hardening soft magnetic stainless steel with good properties of 227 HV after solution treatment, 372 HV and a coercive force of 64 A/m after aging treatment was realized.

Machinability of Selective Laser Melted 17-4 PH Stainless Steel in Turning Process

This paper investigates the machinability of Selective Laser Melted 17-4 Precipitation Hardening Stainless Steel (17-4 PH SS). For this purpose, turning tests were carried out on wrought and SLM specimens and the results were compared. Wear tool, surface roughness, surface integrity and chip morphology were analyzed for both materials. For the tested cutting conditions, the machinability of the SLM material was inferior to that of the commercial material. Dissimilarities in machinability between both materials are a consequence of variations in their microstructures resulting from the manufacturing process.

Improving mechanical properties and isotropy of laser DED 17-4 PH stainless steel by combining ultrasonic vibration

Nanoscale Cu-rich precipitates (CRPs) are important to achieve high strength for the extensively applicated 17-4 precipitation hardening (PH) stainless steel (SS). Traditionally, the precipitation of CRPs requires high-temperature and long-time heat treatments. In this work, ultrasonic vibration (UV) was introduced into the laser direct energy deposition (L-DED) process to fabricate 17-4 PH deposits, a large density of nanoscale CRPs was directly and uniformly precipitated during depositing, the microstructure was also improved. Consequently, the mechanical properties of the deposit, both at the vertical and horizontal directions, were enhanced remarkably compared with those of the deposit manufactured by normal DED. Especially, the elongation was improved significantly, reached to around 20%. Moreover, the isotropy of mechanical property was improved, all the counterpart mechanical properties of the UV-DED sample were nearly close. This investigation supplied a potential method to mitigate the post heat treatments during fabricating 17-4 PH with L-DED.

High Potential Inhibition of the 17-4 PH Stainless Steel in the Presence of Nitrate and Evolution of Metastable Pitting

In the present investigation, the potential effect of nitrate ions’ inhibition on pitting corrosion of 17-4 precipitation hardening (PH) stainless steel in different chloride and nitrate concentrations was investigated using potentiostatic and potentiodynamic examinations. Furthermore, metastable pitting was examined in depth to clarify nitrate inhibition. The results demonstrate that adding nitrate as an inhibitor to the chloride-containing solution decreases the metastable pit peak current, stability products, and pit radius. In contrast, no significant difference between metastable pit lifetime was observed in the presence and absence of nitrate in the chloride solution. It was also shown that introducing 17-4 PH stainless steel to the high potential resulted in the repassivation of the surface, even if the nitrate/chloride concentration ratio was less than the critical value needed for nitrate inhibition. Overall, nitrate ions have an inhibition effect in the chloride solution. Especially at very high potential, whether the nitrate concentration was less than the critical value or not, nitrate forces the surface to get repassivated.

Study of multi-layer deposition in L-DED process for 15–5 PH stainless steel: A numerical and experimental approach

The present work investigates the multilayer deposition of 15–5 Precipitation Hardening (PH) stainless steels via Laser based Direct Energy Deposition (L-DED). The laser power and scanning speed are kept constant at 450 W and 5 mm/s with energy density of 75 J/mm2. At present, very limited literatures are available on the thermal behavior of 15–5 PH multi-layer deposition in relation with its micro structural characteristics. The complex thermal phenomena include repetitive heating and cooling cycles were simulated using 3-D multi-physics model. Through this numerical investigation temperature data of the specific region was captured and correlated with the micro structural morphology. The 15–5 PH multilayer deposition showed clear columnar grains and ferrite transformation. The XRD result revealed that austenite (ɣ) phase fraction increased with the increase in layer deposition. The IPF phase fraction map shows 1.5 % increase in retained austenite phase fraction with each layer addition. In 15–5 PH multilayer deposition the grains are primarily oriented along the 〈101〉/〈001〉 direction and with addition of layer High angle grain boundary (HAGB) fraction increases. The detailed texture analysis shows that as the layer increases, there is formation of brass, copper, shear and rotated goss structure. Similarly, it exhibited proportional rise in hardness due to its δ-ferrite dominating phases and higher HAGB percentage.

Optimization of the dilution parameters to improve wear resistance of laser cladding 15-5PH steel coating on U75V pearlitic steel

The dilution effect of the substrate during laser cladding is inherent, and controlling the dilution degree paves the way for the microstructural design of coatings with excellent mechanical properties. In this study, low-carbon 15–5 precipitation hardening (PH) stainless steel was selected as the cladding material on the surface of U75V pearlitic railway steels to improve wear performance. By altering the processing parameters, the cracking susceptibility, microstructures, and wear resistance of coatings with different dilution rates were investigated thoroughly. The results indicated that with the increase of laser beam power, the dilution effect of the eutectoid steel substrate enhanced obviously, resulting in the composition deviation of coatings. Microhardness of the coatings increased with the dilution rate but the excessive dilution effect led to severe cracking defects. The optimized cladding coating with a dilution rate of about 48 % exhibited a microhardness of 490 HV, and the wear tests showed that the wear volume decreased by about 85 % when compared to that of the U75V substrate. Microstructural characterization revealed that the increased carbon content induced by dilution, resulting in martensite hardening and carbides precipitation, was responsible for these performance improvements. This work demonstrated the potential of coating strengthening by dilution control, which expands the option for surface modification materials for railways.

Numerical Prediction of Microstructure Evolution of Small-Diameter Stainless Steel Balls during Cold Skew Rolling.

The wear resistance and hardness of stainless steel (SS) balls formed by cold skew rolling are effectively improved due to the change in internal microstructure. In this study, based on the deformation mechanism of 316L stainless steel, a physical mechanism-based constitutive model was established and implemented in a subroutine of Simufact to investigate the microstructure evolution of 316L SS balls during the cold skew rolling process. The evolution of equivalent strain, stress, dislocation density, grain size, and martensite content was studied via simulation during the steel balls' cold skew rolling process. The corresponding skew rolling experiments of steel balls were carried out to verify the accuracy of the finite element (FE) model results. The results showed that the macro dimensional deviation of steel balls fluctuates less, and the microstructure evolution agrees well with the simulation results, which proves that the established FE model has high credibility. It shows that the FE model, coupled with multiple deformation mechanisms, provides a good prediction of the macro dimensions and internal microstructure evolution of small-diameter steel balls during cold skew rolling.

Evading strength-ductility trade-off in directed energy deposited precipitation hardenable stainless steels: A pathway through precipitation kinetics modeling, design of heat treatment, and evolution of clusters

The kinetics of precipitation and austenite reversion phase transformations in an as-built arc-directed energy deposition (arc-DED) PH13–8Mo stainless steel are studied. The material in the as-built condition is analyzed using the differential scanning calorimetry (DSC) technique to determine the fraction of transformed precipitates and reverted austenite at any time/temperature during non-isothermal transformations. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) kinetic equation is employed to model the transformed phase volume fraction during precipitation and austenite reversion processes. Using the non-isothermal phase transformation kinetics, an isothermal kinetics model is developed. The isothermal kinetics modeling results are then employed to design and develop direct aging heat treatments to enhance the hardness and strength of arc-DED-PH13–8Mo steel. The developed heat treatment results in the concurrent enhancement of strength and ductility in the material. Such an achievement is a result of the evolution of nano-scaled β-NiAl clusters. It appears that the formation of β-NiAl clusters increases the dislocation storage capability in the material, resulting in the evasion of the strength-ductility trade-off. The results of the current study provide great insight into the crucial role of clusters in the strength-ductility trade-off in PH stainless steels.

Additive manufactured continuum mechanisms based on shape-programmable and micro-sized building blocks

ABSTRACT Micro-additive manufacturing techniques have the potential to meet the demand for miniaturised functional components for minimally invasive surgical instruments. These techniques create monolithic, compliant mechanisms with micro-sized free-form structures that can be tailored to patient-specific surgical procedures. The automated design synthesis of the mechanisms using building blocks results in structures that are shape-programmable. This is achieved through an algorithmic-based computational workflow, which automatically converts user-specified 2D and 3D curves into discrete curve segments. The actuated motion of the mechanisms can be designed to move in a specific way, both forwardly and inversely. The mechanisms are manufactured using micro-laser powder bed fusion and hardenable stainless steel 17-4 PH. By carefully selecting the process parameters, it is possible to 3D-print micro-sized features such as a compliant beam thickness of 80 μm and an actuation hole of 100 μm. Both 2D planar curved mechanisms and 3D spatial curved mechanisms have been implemented and experimentally validated.

XFEM Analysis of Strain Rate Dependent Mechanical Properties of Additively Manufactured 17-4 Precipitation Hardening Stainless Steel

AbstractAdditively manufactured (AM) specimens of 17-4 precipitation hardening (PH) stainless steel (SS) corresponding to the three-point bend test, compact tension test, and single edge cracks were analyzed using the extended finite element method (XFEM) approach. A two-dimensional and three-dimensional elastic-plastic simulation were conducted using “abaqus 6.14” software based on the experimental results and validated with the simulation results. In XFEM, the partition of unity was used to model a crack in the standard finite element mesh. Based on simulation results, the present study compares the mechanical properties of AM 17-4 PH stainless steel samples with those of wrought 17-4 PH samples. Stress intensity factor and J integral were used to measure fracture toughness of the specimens. The change in fracture toughness with strain rate was evaluated by simulating two-dimensional compact tension specimens. The presence of defects such as pores resulting from entrapped gas, un-melted regions, and powder particles resulting from lack of fusion were the main reasons for lower elongation to failure of laser powder bed fusion (L-PBF) produced 17-4 PH SS reported in the literature.

Fatigue crack growth rate and fracture toughness evaluation of 15-5 precipitation hardening stainless steel fabricated by laser powder bed fusion process

The present article reports the fatigue crack growth (FCG) rate and fracture toughness of 15-5 Precipitation Hardening (PH) Stainless Steel (SS) specimens fabricated by Laser powder bed fusion (L-PBF) additive manufacturing (AM) technique. The effect of notch orientation (parallel and orthogonal in reference to build direction) on FCG and fracture toughness is investigated. The FCG and fracture toughness data of L-PBF specimens in the solution annealed (SA) and SA + aged condition were compared to that of wrought 15-5 PH SS. It was found that dependence of FCG on the build orientation was insignificant up to the stress intensity factor range (ΔK) of 30 MPa√m, and beyond that, a higher FCG rate is recorded for L-PBF specimens. The SA specimens were found to have a slightly lower FCG rate at Paris region as compared to SA + aged specimens. The fractography showed predominantly transgranular mode of crack growth, with presence of fine pores and signature of plastic deformation at higher stress intensity. The crack branching and zig-zag path was observed for the L-PBF specimens with notch orientation orthogonal to build direction (horizontal notch). The fracture toughness (K1C) of horizontally notched L-PBF fabricated 15-5 PH SS specimens in aged condition showed higher values (51–62 MPa√m) as compared to vertical (parallel) notched specimens (40–46 MPa√m). The observed fracture toughness values of L-PBF 15-5 PH SS specimens appear to be significantly lower than the reported fracture toughness values of wrought 15-5 PH SS material.

Surface finish, microhardness and microstructure of laser metal deposited 17-4PH stainless steel

Laser metal deposition is a metal-based additive manufacturing technology. It is a very sensitive and complex process because of the different process parameters involved and the interrelations between these parameters. A thorough understanding of the underlying physics of the process is essential in developing a comprehensive database of the properties of materials processed with this technology. The main objective of this study was to investigate the effect of laser power on a laser-deposited 17-4 precipitation hardenable stainless steel alloy. The as-built microstructure, phase composition, microhardness and surface finish were analysed. The results show that a defect-free sample with good metallurgical bonding and minimal dilution can be produced using high laser power in the range 1400–2600 W and a scanning speed of 0.6 m/s. The microstructure in the clad layer was dominated by martensite and an improvement in surface finish and maximum hardness was observed with increased laser power. Significance: To fully benefit from the additive manufacturing technology, a comprehensive database of the material properties of alloys produced with this technology is required. This study expands on the body of knowledge related to the additive manufacturing of a 17-4PH stainless steel alloy, particularly highlighting the possibility of producing fully dense parts using higher laser power and scanning speed. These two parameters could significantly reduce the build time.

On the microstructure and texture evolution in 17-4 PH stainless steel during laser powder bed fusion: Towards textural design

Additive Manufacturing (AM) of metals allows the production of parts with complex designs, offering advanced properties if the evolution of the texture can be controlled. 17-4 precipitation hardening (PH) stainless steel is a high strength, high corrosion resistance alloy used in a range of industries suitable for AM, such as aerospace and marine. Despite 17-4 PH being one of the most common steels for AM, there are still gaps in the understanding of its AM processing–structure relationships. These include the nature of the matrix phase, as well as the development of texture through AM builds under different processing conditions. We have investigated how changing the laser power and scanning strategy affects the microstructure of 17-4 PH during laser powder bed fusion. It is revealed that the matrix phase is δ-ferrite with a limited austenite presence, mainly in regions of the microstructure immediately below melt pools. Austenite fraction is independent of the printing pattern and laser power. However, reducing the time between adjacent laser passes during printing results in an increase in the austenite volume fraction. Another effect of the higher laser power, as well as additional remelting within the printing strategy, is an increase in the average grain size by epitaxial ferrite grain growth across multiple build layers and the development of a mosaic type microstructure. Changes to the scanning strategy have significant impacts on the textures observed along the build direction, while 〈100〉 texture along the scanning direction is observed consistently. Mechanisms for texture formation and the mosaic structure are proposed that presents a pathway to the design of texture via AM process control.

Microstructure and mechanical properties of high-efficiency laser-directed energy deposited 15-5PH stainless steel

Microstructural and mechanical characteristics of 15-5 precipitation hardening (PH) stainless steel fabricated by laser-directed energy deposition (LDED) were investigated thoroughly for high-efficiency deposition. Different from the typical fully martensitic microstructure in wrought counterparts, high-power LDEDed 15-5PH stainless steel in as-deposited condition mainly consists of lath martensite, skeletal δ-ferrite with small amounts of retained austenite and NbC carbides. Formation mechanism of δ-ferrite was proposed based on the combined effects of Cr element segregation and rapid cooling that suppressed the mesothermal austenitic diffusion transformation during LDED process. Besides, the presence of δ-ferrite gives rise to the precipitation of two-scale Cu-rich precipitates after direct aging treatment. The size of CRPs in δ-ferrite (~28.6 nm) is obviously larger than that in martensite (~11.7 nm). The coarsening CRPs in δ-ferrite could be attributed to the surrounding Cu and Ni-depleted conditions. Due to the low original microhardness of δ-ferrite (5.85 ± 0.77 GPa) and the coarsening CRPs inside, relevant tensile tests indicate the yielding and tensile strength of high-efficiency LDEDed 15-5PH stainless steel drops apparently. The present study reveals the possibility of δ-ferrite formation in LDEDed PH stainless steel, which could provide an experimental basis for the optimization of high-efficiency additive process.