Ni Martensitic Stainless Steel Research Articles

Effect of low temperature intercritical heat-treatment on stable crack growth behavior in 13% Cr-4% Ni martensitic stainless steel multipass weldments

Tension and fracture toughness JIc tests were performed on both as-welded and heat-treated conditions. The softer tempered martensitic matrix and the transformation-induced plasticity (TRIP) of reformed austenite improved significantly the strain hardening behavior of the heat-treated material. These matrix characteristics were found to retard micro-void nucleation during straining. The onset of necking, the elongation at fracture, and the fracture toughness JIc were improved by 55%, 28% and 90%, respectively. These results provide an opportunity to document how the matrix mechanical properties influence the fracture behavior of material which rupture is controlled by the nucleation of micro-voids.

Effects of Cyclic Heat Treatment on Microstructure and Mechanical Properties of 13%Cr-4%Ni Martensitic Stainless Steel

The present work investigates the effects of thermal cycling heat treatment (TCHT) on microstructure and mechanical properties of 13%Cr-4%Ni martensitic stainless steel (13-4MSS). As-received 13-4 MSS was subjected to three different thermal cycling schedules by using a thermo-mechanical simulator (Gleeble 3800). The evolved microstructures were studied at three cycles for each thermal cycling schedule with the help of optical, scanning electron microscopy, x-ray analysis, and transmission electron microscopy. Hardness and notched tensile tests were conducted to further characterize the as-received and processed specimens. This cyclic treatment raised the hardness to 413 HV (for TCHT at 950 °C) from 274 HV (for as-received). A 42% (for TCHT at 950 °C) and 39% (for TCHT at 1050 °C) hike in ultimate tensile strength (UTS) with a slight decline in ductility was observed as compared to the UTS and ductility of as-received steel. The evolved microstructure, dislocation density, and the refinement of martensitic blocks attributed to the enhanced hardness and UTS. The coarsening of laths (observed at 1050 °C) and bimodal lath structure (observed at 950 °C) were also found to control the mechanical properties of the present steel.

Through‐Thickness Residual Stresses, Microstructure, and Mechanical Properties of Electron Beam‐Welded CA6NM Martensitic Stainless Steel after Postweld Heat Treatment

In this study, the integrity of electron beam‐ (EB‐) welded CA6NM—a grade of 13% Cr‐4% Ni martensitic stainless steel—was assessed through the entire joint thickness of 90 mm after postweld heat treatment (PWHT). The joints were characterized by examining the microstructure, residual stresses, global mechanical properties (static tensile, Charpy impact, and bend), and local properties (yield strength and strain at fracture) in the metallurgically modified regions of the EB welds. The applied PWHT tempered the “fresh” martensite present in the microstructure after welding, which reduced sufficiently the hardness (<280 HV) and residual stresses (<100 MPa) to meet the requirements for hydroelectric turbine assemblies. Also, the properties of the EB joints after PWHT passed the minimum acceptance criteria specified in ASME sections VIII and IX. Specifically, measurement of the global tensile properties indicated that the tensile strengths of the EB welds in the transverse and longitudinal directions were on the same order as that of the base metal (BM). Evaluation of the local tensile properties using a digital image correlation (DIC) methodology showed higher local yield strengths in the fusion zone (FZ) and heat‐affected zone (HAZ) of 727 MPa and 740 MPa, respectively, relative to the BM value of 663 MPa. Also, the average impact energies for the FZ and HAZ were 63 J and 148 J, respectively, and attributed to the different failure mechanisms in the HAZ (dimples) versus the FZ (quasi‐cleavage consisting of facets and dimples). This study shows that the application of PWHT plays an important role in improving the weld quality and performance of EB‐welded CA6NM and provides the essential data for validating the design and manufacturing process for next‐generation hydroelectric turbine products.

Mehanical Properties of Electron Beam Welded Joints in Thick Gage CA6NM Stainless Steel

Design of hydroelectric turbine components requires high integrity welds (without detectable volumetric defects) in heavy gage sections of stainless steel materials, such as ASTM A743 grade CA6NM—a low carbon 13% Cr-4% Ni martensitic stainless steel that is manufactured in cast form. In this work, 90-mm-thick plates of CA6NM were joined using a single-pass autogenous electron beam (EB) welding process and the mechanical properties were evaluated in the as-welded condition to characterize the performance of the joints. The static tensile properties that were evaluated in two directions—transverse and longitudinal to the EB weld seam—demonstrated conformance of the joints with the requirements of the ASME Section IX standard. The Charpy impact energies of the EB welds—measured at −18 °C on samples with V-notch roots located in the fusion and heat-affected zones—met the minimum requirements of 27 J specified in ASME Section VIII standard. In addition, bend tests that were conducted on the entire weld cross section displayed no discontinuities on the tension side of the bent joints. Hence, the developed EB welding process was demonstrated to render high-performance joints and promises key advantages for industrialization, such as cost savings through reductions in consumable material, production time and labor intensity.

Influence of hammer peening on fatigue life of E309L steel used for 13%Cr-4%Ni blade runner repairs

Austenitic stainless steel E309L is frequently used for welded repairs on 13%Cr-4%Ni martensitic stainless steel blade runners. However, very high tensile residual stresses are found in the upper layers of these repairs, thus reducing the fatigue resistance of the repaired area in comparison with the free of residual stress base material. The use of hammer peening to increase the fatigue life of E309L welded on UNS-S41500 base metal plates has been investigated using welded samples replicating in situ blade runner repairs. Residual stress measurements using the contour method on these samples have confirmed the high stresses caused by flux cored arc welding (FCAW) of E309L, leaving stress magnitudes close to the ultimate tensile strength of the base material. Hammer peening of the surface beads effectively creates high compressive stresses at the weld’s surface up to 3mm depth. Alternating bending fatigue tests conducted on hammer peened samples have shown an average life increase of 5.8 times at Sa=330MPa over unpeened samples. Fatigue tests have however shown diminishing returns when overpeening the samples.

Mechanical properties of electron beam welds in heavy-section UNS S41500 martensitic stainless steel

Single pass autogenous butt welds, without detectable volumetric defects, were manufactured by electron beam welding 90-mm-thick UNS S41500—a low carbon 13% Cr-4% Ni martensitic stainless steel typically used for hydroelectric turbine applications. The mechanical properties of the joints were evaluated in the as-welded condition by tensile testing, Charpy V-notch testing, and bend testing. Specifically, static tensile loading was conducted on samples extracted transverse and longitudinal to the weld seam. To determine the impact toughness of the electron beam welded joints, the Charpy tests were conducted on samples with V-notch roots located in the fusion and heat-affected zones at −18 °C. The yield strength, ultimate tensile strength, elongation and toughness of the electron beam welds were found to more than adequately meet the minimum acceptance criteria specified in ASME section VIII and IX. Also, the formability of the welds, examined by bend testing, displayed no discontinuities on the tension side of the bent joints. These promising mechanical properties determined for the as-welded UNS S41500 stainless steel offer considerable prospects for industrializing the electron beam welding process.

Investigation of the evolution of retained austenite in Fe–13%Cr–4%Ni martensitic stainless steel during intercritical tempering

Abstract The microstructure and amount of retained austenite (the austenite remained at room temperature) evolved in Fe–13%Cr–4%Ni martensitic stainless steel during intercritical tempering at 620 °C have been investigated. The amount of retained austenite showed a parabolic trend with increase in tempering time, which can be attributed to the gradual decrease in the thermal stability of the reversed austenite (the austenite formed at high temperature). The influences of chemical composition, morphology of reversed austenite, and mechanical constraints originating from tempered martensite matrix on the thermal stability have been discussed. The precipitation and growth of M 23 C 6 in reversed austenite dilute the carbon concentration in reversed austenite. The spheroidization of lathy reversed austenite during tempering decreases the interfacial energy barrier to the phase transformation of reversed austenite to martensite. Furthermore, the decrease in the strength of martensite matrix lowers the strain energy associated with the transformation of reversed austenite to martensite. All these factors during tempering weaken the thermal stability of reversed austenite and facilitate the phase transformation of reversed austenite to martensite during the cooling step of intercritical tempering.

In situ investigation on the deformation-induced phase transformation of metastable austenite in Fe–13% Cr–4% Ni martensitic stainless steel

Deformation-induced phase transformation (DIPT) of metastable austenite in Fe–13% Cr–4% Ni steel occurs during the macroscopic elastic stage and is accompanied by the yielding of metastable austenite at microscopic level. The DIPT rate is accelerated by yielding the martensite matrix, which increases the available nucleation sites for DIPT by enhancing the plastic deformation in austenite.

Investigation of the mechanical stability of reversed austenite in 13%Cr–4%Ni martensitic stainless steel during the uniaxial tensile test

The mechanical stability of reversed austenite at room temperature in two 13%Cr–4%Ni low carbon martensitic stainless steel samples after different heat treatments has been investigated. The uniaxial tensile tests indicate that the reversed austenite resulting from the one-stage and two-stage intercritical tempering heat treatment have different mechanical stability, which induces distinct strength-ductility balance of the material. Experiments and crystal plasticity finite method simulations reveal that the special grain orientation relationship between the reversed austenite and martensite matrix, besides chemical composition, plays important roles on the mechanical stability of the reversed austenite. It is found that in both samples the Nishiyama–Wassermann or Kurdjumov–Sachs relationship between the reversed austenite and the martensite matrix provides an easy way for the active slip systems in austenite to penetrate the phase boundary to the adjacent martensite. This results in a high mechanical stability of the reversed austenite. In addition, the larger austenite grains in the second-stage tempering sample have higher mechanical stability in the martensitic steel due to the favored austenite stabilizing elements distribution behavior, and the favored stress distribution originating from the intrinsic strengths of the austenite and martensite matrix.

Temperature Dependence of Austenite Nucleation Behavior from Lath Martensite

The temperature dependence of austenite nucleation behavior was investigated in an ultralow carbon 13%Cr–6%Ni martensitic stainless steel. The martensitic structure was partially reversed to austenite by heat treatment at different temperature in (austenite+ferrite) two-phase region. With increasing the reversion temperature, the shape of austenite grains tend to be changed from acicular to granular, and their nucleation site is changed from lath boundaries to prior austenite grain boundaries. The transition of nucleation site was discussed in terms of energetics by considering the increases in interfacial energy and elastic strain energy by formation of an austenite nucleus. The calculation results suggested that lath boundary is more preferential nucleation site rather than prior austenite grain boundary because the increment of elastic strain energy is reduced with lowing the reversion temperature.

Temperature Dependence of Austenite Nucleation Site on Reversion of Lath Martensite

The temperature dependence of austenite nucleation behavior within lath martensitic structure was investigated in an ultralow carbon 13%Cr-6%Ni martensitic stainless steel partially reversed at (austenite + ferrite) two phase region. The shape and nucleation site of the reversed austenite grains were varied depending on the reversion temperature; fine acicular austenite grains frequently formed along the lath boundaries at a temperature lower than 915 K, while the granular ones tended to nucleate mainly on the prior austenite grain boundaries at a higher temperature. In order to explain the temperature dependence of nucleation site transition, the difference in energetics of austenite nucleation between the lath boundary and the prior austenite grain boundary was discussed on the basis of the classical nucleation theory and FEM analysis. The calculation of the changes in interfacial energy and elastic strain for austenite nucleation suggested that the lath boundary acts as more preferential nucleation sites for austenite rather than the prior austenite grain boundary to reduce the increment of elastic strain when the reversion temperature is low.

Residual stress and microstructure in welds of 13%Cr–4%Ni martensitic stainless steel

The objectives of the present study were to characterize the distribution of residual stress and the microstructure changes induced by welding in the heat-affected zone (HAZ) of 13%Cr–4%Ni used in hydraulic turbine fabrication to deduce best practices. To characterize residual stress after welding, hole-drilling, X-ray diffraction (XRD) and the contour method were used on a FCAW 6 passes UNS W41036 (410NiMo) weld deposited on UNS S41500 (13%Cr–4%Ni) stainless steel; the results were put side to side with the weld microstructure and hardness to assess their criticality. Transverse compression was found in and around the last bead of the weld by XRD and hole-drilling. Longitudinal compression stress was found in and around the last bead while longitudinal tension was found near the low-temperature HAZ. The contour method showed that despite high compression in the last bead, high longitudinal tension exists in the rest of the weld and just below the weld. The superposition of both residual stress distribution and results from microstructural characterization shows that in multipass welding of 13%Cr–4%Ni martensitic stainless steel, cracking susceptibility is higher in the weld than in the HAZ, let it be fatigue cracking, environment-assisted cracking or cold cracking during welding. Post-weld heat treatment proved to be very efficient in lowering residual tension found in the first bead and in lowering the hardness of the weld. These results underline the importance of following proper procedures when welding these steels; this being even more true when the assembly is loaded in fatigue.

Effect of Tempering on the Anodic Polarization Behavior of an 11 Cr‐12 Ni Martensitic Stainless Steel

Anodic polarization measurements in and in were made on an 11 Cr‐12 Ni martensitic stainless steel in the as‐rolled, as‐quenched, and quenched and tempered conditions. Structural changes on tempering between 100° and 600°C were evaluated by hardness measurements, light metallography, and transmission electron microscopy. The basic shape of the anodic polarization curves was not significantly affected by cold work (75% reduction) or by the carbide precipitation, recovery and recrystallization which occurred on tempering. The critical potential for pitting was unaffected by tempering and chloride addition. The primary passivation potential became more active on tempering above 300°C for 1 hr. A 600°C temper, accompanied by significant recrystallization, resulted in severe transpassive region pitting when the steel was polarized in the presence of Cl−.