Effect of Cr/Ni equivalent ratio on ductility-dip cracking in AISI 316L weld metals

The ductility-dip cracking (DDC) susceptibility of AISI 316L stainless steel weld metals was examined. Modified flux cored arc welding (FCAW) filler wires were fabricated with various chromium and nickel contents. The solidification mode and delta-ferrite content were determined from the chromium and nickel equivalent ratios (Cr_eq/Ni_eq). Ductility-dip cracking occurred through a grain boundary sliding mechanism in the reheated region of the weld metal in the ferrite at cell and dendrite boundaries (AF mode), and the primary course of DDC formation was associated with the straight migrated grain boundary (MGB) morphology. No DDC was observed in the tortuous MGB due to the pinning effect of delta-ferrite in the continuous network of vermicular type of ferrite (FA mode)/acicular ferrite and continuous austenite network (F mode) weld metals. The DDC at the triple point or the intersection of the MGB showed a creep-like morphology. Severe localized and thermal plastic deformation was observed through the formation of micro-voids when grain boundary sliding was generated in the ductility-dip temperature range under strong restraint conditions.     

钨含量对新型高铬锰氮双相不锈钢Cr29Mn12Ni2N0.6Wx组织和性能的影响

研究了钨含量对新型高铬锰氮双相不锈钢Cr29Mn12Ni2N0.6Wx(x=1,2,3)显微组织、力学性能和耐腐蚀性能的影响。结果表明:Cr29Mn12Ni2N0.6Wx不锈钢固溶处理后具有典型的铁素体+奥氏体双相组织,铁素体含量在45%~60%范围内;随着钨含量的增加,合金中σ相的析出倾向增强,铁素体含量增加,合金的耐腐蚀性能降低,屈服强度和抗拉强度升高;经1 050℃固溶处理30 min后,该系列双相不锈钢中不再有σ相析出,其屈服强度大于650 MPa,抗拉强度大于900 MPa,断后伸长率大于30%,作为高强度资源节约型超级双相不锈钢具有潜在应用前景。     

Effect of 475°C embrittlement on the low cycle fatigue behaviour of lean duplex stainless steels

Lean duplex stainless steels (LDSSs) with lower nickel and molybdenum are less susceptible to suffer spinodal decomposition than standard duplex stainless steels. It is the purpose of this work to study the effect of thermal embrittlement on the low cycle fatigue behaviour of 2 LDSSs with different Cr_eq and Ni_eq. The correlation between the fatigue behaviour and the dislocation structure is attempted. Transmission electron microscopy was used to observe the dislocation microstructure. Additionally, STEM‐EDS technique in conjunction with Vickers microhardness measurements was used to characterize the amplitude of the spinodal decomposition. The results show that the LDSS with lower Cr_eq and Ni_eq values exhibits improved fatigue properties in the as received and aged conditions. Furthermore, it is important to emphasize that with an adequate volume fraction of phases in LDSSs, the ageing treatment leads to an increase in strength without causing a great detriment in low cycle fatigue life.     

A mechanism of ferrite softening in a duplex stainless steel deformed in hot torsion

A mechanism of dynamic softening of ferrite was studied in a 21Cr-10Ni-3Mo austenite/ferrite duplex stainless steel subjected to torsion at a strain rate of 0.7 s⁻¹ at 1200°C. Transmission electron microscopy together with convergent beam electron diffraction were used with major emphasis on the study of misorientations across ferrite/ferrite boundaries. No evidence of discontinuous dynamic recrystallisation involving nucleation and growth of new grains was found within ferrite contrary to some suggestions made in the literature for similar experimental conditions. The softening mechanism has been classified as extended dynamic recovery characterised by a gradual increase in misorientations between neighbouring subgrains that were created by dynamic recovery processes at the earlier stages of deformation. The resulting dislocation substructure was a complex network of subgrain boundaries composed of a mix of higher- and lower-angle walls characterised by misorientation angles not exceeding 20° at a maximum obtained strain of 1.3.     

Considerations for the weldability of types 304L and 316L stainless steel

The susceptibility of austenitic stainless steels to the formation of two distinct weld defects, solidification cracking and lack of penetration, is related to the chemical composition of the base and filler material. The propensity for cracking is determined primarily by the solidification mode and the amount of residual tramp elements such as phosphorous and sulfur. High sulfur levels can lead to weld centerline cracking and heat affected zone (HAZ) cracking while very low sulfur levels (less than ∼50 ppm) in types 304L and 316L are associated with lack of penetration weld defects and a distinct loss in puddle control during fusion welding. A calculated Cr_eq to Ni_eq ratio of 1.52 to 1.9 is recommended to control the primary mode of solidification and prevent solidification cracks in type 304L while the Cr_eq/Ni_eq ratio of 1.42 to 1.9 is recommended for type 316L stainless steel. A lower limit of 50 ppm sulfur is recommended to avoid possible lack of penetration. These ranges should be validated by welding trials for specific weld processes and applications.     

Mechanical stabilisation of retained austenite in δ-TRIP steel

Recent research suggests that extraordinary combinations of strength and ductility can be achieved in the so-called δ-TRIP steels, which contain ferrite, bainite and austenite. A part of the reason for the ductility of almost 40% elongation at a strength of some 900 MPa, is believed to be the optimal stability of the austenite to plastic deformation. We demonstrate here that mechanical stabilisation plays an important role in preserving the austenite to large plastic strains.     

Correlation of austenite stability and ductile-to-brittle transition behavior of high-nitrogen 18Cr-10Mn austenitic steels

Ductile-to-brittle transition behavior of high-nitrogen 18Cr-10Mn austenitic steels containing different contents of Ni, Mo, Cu as well as nitrogen is discussed in terms of austenite stability and associated deformation-induced martensitic transformation (DIMT). Electron back-scattered diffraction and transmission electron microscopy analyses of cross-sectional area of the Charpy impact specimens fractured at −196 °C indicated that the brittle fracture planes were almost parallel to one of {1 1 1} slip planes and some metastable austenites near the fracture surface were transformed to α′-martensite by localized plastic deformation occurring during crack propagation. Quantitative evaluation of deformation-induced martensite together with characteristics of true stress-strain and load-displacement curves obtained from tensile and Charpy impact tests, respectively, supported that DIMT might take place in high-nitrogen austenitic steels with relatively low austenite stability. The occurrence of DIMT decreased low-temperature toughness and thus increased largely ductile-to-brittle transition temperature (DBTT), as compared to that predicted by empirical equations strongly depending on nitrogen content. As a result, the increased DBTT could be reasonably correlated with austenite stability against DIMT.     

Hot working behavior of 2205 austenite-ferrite duplex stainless steel characterized by constitutive equations and processing maps

The hot deformation characteristics of the 2205 duplex stainless steel were analyzed using constitutive equations and processing maps. The hot compression tests were performed at temperature range of 950-1200 °C and strain rate of 0.001-1 s⁻¹. Flow stress was modeled by the constitutive equation of hyperbolic sine function. However, the stress exponent and strain rate sensitivity were different at low and high deformation temperatures where austenite and ferrite are dominant, respectively. It was recognized that strain at the peak point of flow curve increases with the Zener-Hollomon parameter, Z, at low temperature deformation while at high temperature deformation it actually decreases with Z. The power dissipation map, instability map and processing map were developed for the typical strain of 0.3. It was realized that dynamic restoration mechanisms could efficiently hinder the occurrence of flow instability at low and medium strain rates. Otherwise, the increase in strain rate at low and high temperatures could increase the risk of flow instability.     

Dynamic behaviour of 304 stainless steel during high Z deformation

The deformation behaviour of 304 stainless steel was investigated over a range of strain rates spanning those from typical laboratory conditions through to those more commonly experienced in industry. This study examines the effect of the Zener-Hollomon (Z) parameter on the shape of the deformation flow curves. At Z values typical of laboratory experiments the flow curves display a behaviour indicative of dynamic recrystallization (DRX). A clear peak can be seen in the curve, but as the Z value increases (increasing strain rate/decreasing temperature) the shape of the curve changes to a ‘flat-top’ behaviour which has traditionally been used to indicate that no DRX is occurring and that dynamic recovery (DRV) is the only softening mechanism operating. Examination of the corresponding work hardening curves (θ = δσ/δ?) and deformation microstructures suggests that some level of dynamic recrystallization has occurred. The results suggest that at the higher strain rates the main mechanism for DRX is only the formation from prior austenite grain boundary bulges and that subsequent layers of DRX grains are very difficult to form. The overall deformation textures were also found to be independent of the strain rate.     

Importance of deformation induced ferrite and factors which control its formation

Abstract

The influence of strain, strain rate, temperature, and grain size on the formation of deformation induced ferrite has been examined. Deformation induced ferrite forms very readily in both fine and coarse grained steels and much more rapidly than the ferrite from strain free austenite. Very small strains are sufficient to induce the production of such ferrite and the temperature range over which it appears spans from just below the Ae₃ temperature down to the undeformed Ar₃ temperature. Although it forms readily in both coarse and fine grained steels, the volume fraction produced is sensitive to the austenite grain size. In coarse grained steels, deformation at low strain rates is concentrated along the grain faces; extensive dynamic recovery occurs, which is why the ferrite remains soft, so that only thin ferrite films are able to form. At higher strain rates, work hardening takes place so that the strength of the ferrite at high strains approaches that of the austenite. Under these conditions, the deformation is propagated towards the centres of the austenite grains and larger volume fractions of deformation induced ferrite are able to form. In fine grained steels, the flow stress in the austenite grain boundary region is increased, so that when ferrite first forms, a considerable amount of work hardening takes place, which strengthens the ferrite. When combined with the increased number of triple points present in the material, the increased work hardening promotes spreading of the deformation, with the result that larger volume fractions of ferrite are produced, even at low strains and strain rates.     

Grain-boundary diffusion and precipitate trapping of hydrogen in ultrafine-grained austenitic stainless steels processed by high-pressure torsion

This study was conducted to clarify the effects of grain boundaries and precipitates on room-temperature hydrogen transport in two types of austenitic stainless steels with ultrafine-grained structures produced by high-pressure torsion (HPT) and subsequent annealing. The grains in the Fe-25Ni-15Cr (in mass%) alloy containing Ti and the Fe-25Cr-20Ni alloy were refined by the HPT-processing to ∼150 and ∼85 nm, respectively. The high-temperature annealing after the HPT processing led to the precipitation of η-Ni₃Ti for the former and σ-FeCr for the latter. In the HPT-processed specimens, hydrogen diffusivity was enhanced through short-circuit diffusion because of the increased population of grain boundaries in comparison with the increased opportunity of hydrogen trapping on dislocations. As for the post-HPT-annealed specimens having the precipitates, the hydrogen diffusion was hindered by the hydrogen trapping on η-Ni₃Ti precipitates, but was not affected by σ-FeCr precipitation. This depends on the affinity between hydrogen and constituting elements.     

Tensile deformation of a duplex Fe-20Mn-9Al-0.6C steel having the reduced specific weight

The room temperature deformation characteristics of a duplex Fe-20Mn-9Al-0.6C steel with the reduced specific weight of 6.84 g/cm³ in the fully solutionized state were described in conjunction with the deformation mechanisms of its constituent phases. The phase fraction was insensitive to annealing temperature in the range of 800-1100 °C. The ferrite grain size was also nearly unaltered but the austenite grain size slightly increased with increasing annealing temperature. This revealed that there is little window to control the microstructure of the steel by annealing. The steel exhibited a good combination of strength over 800 MPa and ductility over 45% in the present annealing conditions. Ferrite was harder than austenite in this steel. Strain hardening of both phases was monotonic during tensile deformation, but the strain hardening exponent of austenite was higher than that of ferrite, indicating the better strain hardenability of austenite. In addition, the strain hardening exponent of austenite increased but that of ferrite remained unchanged with increasing annealing temperature. The overall strain hardening of the steel followed that of austenite. Considering element partitioning by annealing, the stacking fault energy of austenite of the steel was estimated as ∼70 mJ/m². Even with the relatively high stacking fault energy, planar glide dominantly occurred in austenite. Neither strain induced martensite nor mechanical twins formed in austenite during tensile deformation. Ferrite exhibited the deformed microstructures typically observed in the wavy glide materials, i.e. dislocation cells. The mechanical properties of the present duplex steel were compared to those of advance high strength automotive steels recently developed.     

Study of hot deformation behaviour of 2205 duplex stainless steel through hot tension tests

The hot working behaviour of duplex stainless steels has been studied in the literature mainly through hot torsion or hot compression tests. The aim of this paper is to investigate the hot deformation behaviour of a duplex stainless steel type 2205 (F51) through hot tension tests, which are easier to carry out and can offer some additional information about the maximum sustainable strain in tension before starting of irreversible damage. In fact, even under the uniaxial compressive action of a forging press, tensile stresses may develop in some zones of the forged product. These zones become most critical and the knowledge of the limiting conditions are important. Three different test temperatures were selected, namely 900, 1000, and 1100 °C. The hot working behaviour was characterised by fitting the mean flow stress, strain rate, and temperature relationship with the hyperbolic sine function defined by Sellars and Tegart. An activation energy Q equal to 430 kJ/mol was obtained for plastic straining at high temperature. It corresponds to values, obtained with hot torsion or hot compression tests, reported in the literature where a large variation in activation energy is also found for duplex stainless steels. It was explained by considering the intrinsic two-phase nature of the investigated steel. The ductility in the different hot working conditions was characterised by the true strain at the onset of damage formation under prevailing tensile stress conditions. The role of the microstructure as well as of damage formation during deformation on the shape of the flow stress curves was analyzed.     

Strain induced martensitic transformation of Fe-20Cr-5Mn-0.2Ni duplex stainless steel during cold rolling: Effects of nitrogen addition

The effects of nitrogen addition on the strain induced martensitic transformation (SIMT) behavior of duplex stainless steel (D-STS) with the low nickel content were examined in a wide range of strain by means of cold rolling. Nitrogen of 0.1, 0.2 and 0.3 wt.% was added into Fe-20Cr-5Mn-0.2Ni D-STS (in wt.%) and cold rolling was conducted up to the effective strain of ∼1.45 after annealing at 1000 °C for 30 min. In the as-annealed state, the austenite fraction increased with increasing the N content. Regardless of the N content, the ferrite grain size was coarser than that of austenite. The stacking fault energy of austenite of the present D-STSs inferred by the element partitioning analysis was low enough so that SIM transformation is available. Accordingly, during cold rolling, SIMT occurred in austenite with a sequential manner of austenite → ? martensite → α′ martensite with increasing strain. By contrast, the deformed microstructure of ferrite was dominated by dislocation cells. The SIM fraction, which was normalized with reference to the initial austenite fraction in order to eliminate its effect, increased with increasing the N content. Along with the present results, the factors influencing the SIMT kinetics in the present D-STSs with the different N content were discussed.     

The effect of low temperature annealing on the mechanical behavior of cold rolled dual-phase Twinning-Induced Plasticity steel

In recent years, Twinning-Induced Plasticity (TWIP) steels with high specific strength have been developed to mainly address the unsaturated demands of transportation industries for weight reduction. To achieve the exclusive mechanical properties of TWIP steels, the understanding of their thermomechanical processing (TMP) behavior is highly necessitated. In the present work, the influence of cold rolling and post-annealing treatments on the mechanical behavior of a new dual phase (γ + α) TWIP steel have been studied. The microstructural studies indicated the presence of deformation twins in the deformed state of material. Annealing the as-rolled experimental alloy could result in the formation of Widmanstätten austenite within the ferrite grains at 500 °C. The nearly constant yield stress at high annealing durations was attributed to the opposite effects of recovery and Widmanstätten austenite formation.     

Grain refinement of biomedical Co-27Cr-5Mo-0.16N alloy by reverse transformation

Advanced grain refinement of a biomedical Ni-free Co-27Cr-5Mo-0.16N alloy without hot or cold plastic deformation was successfully achieved by a reverse transformation from a lamellar (hcp + Cr₂N) phase to an fcc phase. The technique consisted of a two-step heat treatment. First, the solution-treated specimen was subjected to isothermal aging at 1073 K for 90 ks, forming a lamellar structure of hcp and Cr₂N phases. Then, the aged specimen having a completely lamellar microstructure was reverse-treated at temperatures from 1273 to 1473 K, where the fcc phase is stable. The resultant grains were approximately 1/10 of their initial size. Moreover, tensile testing after reverse transformation showed excellent strength with good ductility compared to samples examined before the reverse transformation. Our results will contribute to the development of biomedical Ni-free Co-Cr-Mo-N alloys with refined grain size and good mechanical properties, without requiring any hot workings.     

Resistance upset butt welding of austenitic to martensitic stainless steels

In this research, joining austenitic to martensitic stainless steels and effect of welding power on microstructure and mechanical properties of the joint were investigated. Microstructure of the weld was studied using optical microscopy (OM) and scanning electron microscopy (SEM). Energy dispersive spectroscopy (EDS) linked to SEM was used to determine chemical composition of phases and distribution of chromium (Cr), nickel (Ni) and iron (Fe) at the joint interface. Microhardness and tensile strength tests were performed. Finally fracture surface of samples were studied by SEM. Results showed that an interlayer composed of 80% ferrite and 20% martensite has formed at the joint interface and there were three different zones in the heat affected zone (HAZ) of two steels. Different forms of austenite phase including widmanstatten austenite (W_γ), allotriomorphic austenite (A_γ) and intergranular austenite (I_γ), delta ferrite (δ-ferrite) and chromium carbide (Cr₂₃C₆) have formed in the HAZ of austenitic stainless steel. Fractography of tension samples indicated that in all samples fracture occurred in austenitic stainless steel HAZ. The strength and hardness of the joint increased and HAZ length decreased with increasing of welding power.     

Sintered duplex stainless steels from premixes of 316L and 434L powders

Duplex austenite–ferrite stainless steels were prepared from the premixes of 316L and 434L stainless steel atomized powders. Pronounced densification was observed after 1350°C sintering in hydrogen. 316L-60w/o 434L steel composition exhibited maximum transverse rupture strength, while 40 and 60w/o 434L containing compositions showed total immunity in 1N H₂SO₄ even after a exposure time of 360 h. Anodic polarization curves also suggest high-corrosion resistance of those two compositions. Magnetic coercivity decreased with increase in sintering temperature while magnetic saturation follows the reverse trend. Wear resistance of the duplex stainless steels under sliding condition was in between the straight steels.     

Microstructure, martensitic transformation and superelasticity of Ti49.6Ni45.1Cu5Cr0.3 shape memory alloy

The aim of this study is to investigate the microstructure, martensitic transformation behavior, shape memory effect and superelastic property of Ti_49.6Ni_45.1Cu₅Cr_0.3 alloy, with Cu and Cr substituting for Ni. After annealing, the alloy showed single step A-M/M-A transformations within the whole annealing temperature range of 623 K to 1273 K even in the presence and Ti₂(Ni, Cu) precipitates. With the increase of the annealing temperature, the transformation temperatures exhibited three stages: increasing from 623 K to 873 K, decreasing from 873 K to 1023 K and unchanging from 1023 K to 1273 K. Meanwhile, the critical stress for stress induced martensitic (SIM) transformation decreased to a minimum value and increased after that, exhibiting a V shape curve. The alloy annealed at 623, 773 and 923 K exhibited shape recovery ratio more than 90% when the deformation strain was below 20%.     

Effect of hydrogen on orientation dependence of critical shear stress and mechanism of straining in single crystals of stable stainless steel

The effect of hydrogen on the critical shear stress (τ_cr), dislocation structure, and mechanisms of straining (slip vs. twinning) in single crystals of Fe-26Cr-32Ni-3Mo (wt %) austenite stainless steel has been studied. It is shown for the first time that hydrogenation leads to the appearance of the orientation dependence of τ_cr, which is absent in the initial hydrogen-free crystals. This dependence of τ_cr is related to a decrease in the stacking fault energy, which determines a change of the mechanism of straining from slip to twinning.