Suppressing type IV failure via modification of heat affected zone microstructures using high boron content in 9Cr heat resistant steel welded joints

Abstract

Creep rupture strength at 923 K and microstructural evolution of welded joints have been investigated for high boron–low nitrogen–9Cr heat resistant steels developed at the National Institute for Materials Science (Japan). Welded joints were prepared from plates containing 47–180 ppm boron using gas tungsten arc welding and Inconel type filler metal, and showed superior creep properties to those of welded joints of conventional high chromium steels such as P92 and P122. No type IV failure was observed in the boron steel welded joints. A large grained microstructure was observed in the heat affected zone heated to Ac ₃ (Ac ₃ HAZ) during welding, whereas the grains are refined at the same location in conventional steel welded joints. The simulated Ac ₃ HAZ structures of the boron steels have a creep life almost equal to that of the base metal. Large grained HAZ microstructures and stabilisation of M₂₃C₆ precipitates are probable reasons for suppression of type IV failure and improved creep resistance of the boron steel welded joints.     

The Microstructure and Gamma Prime Distributions in Inertia Friction Welded Joint of P/M Superalloy FGH96

A gamma prime (γ′) precipitation (~35% in volume)-hardened powder metallurgy (P/M) superalloy FGH96 was welded using inertia friction welding (IFW). The microstructure and γ′ distributions in the joints in two conditions, hot isostatic pressed state and solution-treated and aged state, were characterized. The recrystallization of grains, the dissolution and re-precipitation of γ′ in the joints were discussed in terms of the temperature evolutions which were calculated by finite element model analysis. Regardless of the initial states, fully recrystallized fine grain structure formed at welded zone. Meanwhile, very fine γ′ precipitations were re-precipitated at the welded zone. These recrystallized grain structure and fine re-precipitated γ′ resulted in increasing hardness of IFW joint while making the hardness dependent on the microstructure and γ′ precipitation.     

Features of austenite formation in low-carbon steel upon heating in the intercritical temperature range

The features of austenite formation upon continuous heating of low-carbon steel at the rates of 90–0.15 K/s in the intercritical temperature range (ICTR) have been studied. It has been found that, in the initially high-tempered, initially quenched, and initially cold-deformed steel, the α → γ transition in the ICTR consists of three stages. The thermokinetic diagrams of the austenite formation with the indication of the positions of the critical points Ac₁ and Ac₃ and also of the temperature ranges of the development of each identified stage of the α → γ transformation have been constructed. A complex of structural studies has been carried out, and a scheme of the austenite formation upon continuous heating at a rate of 90 K/s in the ICTR for the initially high-tempered steel, initially quenched steel, and initially cold-deformed low-carbon steel has been suggested, which reflects all stages of this process.     

Analysis of factors affecting type IV cracking in welded joints of high chromium ferritic heat resistant steels

Abstract

In the present work, creep tests on single pass welded joints were carried out to investigate the factors affecting the deterioration of the creep strength of weldments of high Cr heat resistant steels. In creep tests at a temperature of 923 K and stresses of 90 and 120 MPa, creep fracture occurred in the fine grained heat affected zone (FGHAZ) and was identified as type IV cracking. It was found that a peak temperature between Ac₁ and Ac₃ in the FGHAZ caused the observed creep property deterioration. The factors identified as leading to the deterioration are a fine grain structure, low hardness, and large precipitates. Further investigations confirmed the influence of the fine grain structure as a critical factor decreasing the creep rupture time. The hardness difference between the FGHAZ and the base metal is insignificant, especially at 923 K. However, SEM investigations revealed that many large precipitates are distributed on the boundaries of the martensite laths and prior austenite grain boundaries. It is considered that the large precipitates also have a significant influence on the creep strength deterioration.     

镁/镀锌钢板CMT熔钎焊连接机制分析

采用CMT焊对AZ31B镁合金和HDG60镀锌钢异种材料进行熔钎焊.在试验中,采取了搭接焊的方式,通过调整焊接参数得到最佳焊接成形.使用扫描电子显微镜(SEM)、能谱分析(EDAX)、电子探针、X射线衍射(XRD)及拉伸试验对焊接接头微观连接机制及性能进行研究.结果表明,镁和镀锌钢能够形成良好的搭接接头.焊接接头可以分成焊缝区、结合界面、熔合区.结合面主要有Al,Zn,Mg三种元素,主要相有Al₁₂,Mg₁₇,Mg₂Zn₁₁,Al₇Zn₃及少量的MgFeAlO₄复合氧化物.Zn和Al元素对镁钢连接起着关键作用,Zn在焊接接头形成过程中仍有一定的流动作用.在拉伸试验中,焊接接头试样几乎都断裂在熔合区,抗剪强度可达218 MPa.     

Features of nitroquenching of medium-carbon steel during anodic electrolyte–plasma processing

The research described in this paper is devoted to short-term anodic nitriding of grade 45 steel with postquenching (nitroquenching) in the aqueous solution of ammonia and ammonia chloride. The modified layer structure, as revealed, is composed of alternate layers: a surface oxide layer composed of FeO and Fe₃O₄; a layer of dispersed nitrides Fe₄N and Fe_2–3N with retained austenite; next, a martensitic zone composed of nitrogen and carbon; and the initial nitrogen-enriched ferrite–pearlitic structure. It is found that concentrations of electrolyte components and processing conditions affect the formation character and properties of diffusion layers. The possibility is shown for obtaining a nitroquenched layer 130 µm in thickness with a surface microhardness of to 1200 HV with a decrease in roughness R_a from 0.57 to 0.55 µm, R_z from 1.75 to 1.62 µm, and R_max from 5.74 to 4.00 µm.     

Origin of abnormal formation of pearlite in medium-carbon steel under nonequilibrium conditions of heating

The structure and kinetics of the formation of austenite in medium-carbon steel during shortterm heating above the temperature Ac₁ followed by accelerated cooling are analyzed. It has been shown that the abnormal formation of pearlite in steel results from the concentrational and structural inhomogeneity of austenite, as well as the presence of carbide particles in ferrite areas.     

Characterization of Mechanical Properties and Residual Stress in API 5L X80 Steel Welded Joints

The use of high-strength and low-alloy steels, high design factors and increasingly stringent safety requirements have increased the operating pressure levels and, consequently, the need for further studies to avoid and prevent premature pipe failure. To evaluate the possibility of improving productivity in manual arc welding of this type of steel, this work characterizes the mechanical properties and residual stresses in API 5L X80 steel welded joints using the SMAW and FCAW processes. The residual stresses were analyzed using x-ray diffraction with the sin² ψ method at the top and root of the welded joints in the longitudinal and transverse directions of the weld bead. The mechanical properties of the welded joints by both processes were characterized in terms of tensile strength, impact toughness and Vickers microhardness in the welded and shot peening conditions. A predominantly compressive residual stress was found, and shot peening increased the tensile strength and impact toughness in both welded joints.     

Effect of grain size on heat affected zone cracking susceptibility.Study of weldability of Inconel 718 cast alloy (2nd Report)

Summary

It is generally recognised that the flux type of covered electrodes affects the profile and mechanical properties of welds and may well control the fatigue strength of fillet welded joints. Fillet welded joints, however, have seen few systematic investigations. The purpose of this investigation is to improve the fatigue strength of fillet welded joints through appropriate design of the covered electrode flux.

Ten types of covered electrode were trial‐manufactured with variation in the flux system at two levels (to improve the profile of the fillet toe region) and the added alloying element contents of each system at five levels (to vary the hardness of the weld metal). These covered electrodes were used to prepare non‐load‐carrying cruciform fillet welded joints using 570 MPa class high‐strength steel base metal. Fatigue tests (S = σ_max ‐ σ_min, σ_max = σ _Y ) were run with the fillet welded joints. The results obtained may be summarised as follows:

1. The profile of the fillet welds appears to be affected by the flux system and base metal but not by the added alloying elements.

2. The maximum stress concentration factor (K_max) is reduced to K_max ? 1.59 for covered electrodes using an MgCO₃‐MgO‐CaF₂‐iron powder system flux (M' series).

3. The fatigue strength of the fillet welded joints at 2 × 10⁶ cycles is improved to around 143 MPa in the M' series. It can safely be said that there is an inverse correlation between K_max in the toe region and the fatigue strength of the welded joints.

4. Appropriate relations are developed and used to calculate the recovery to the weld metal of added alloying elements. The recovery to the weld metal of the added alloying elements on average gives minima of around 98%Ni, 94%Mo, 85%Cr (which are good), and around 11%B.

     

Quantification of Microtexture at Weld Nugget of Friction Stir-Welded Carbon Steel

Friction stir welding of C-Mn steel was carried out under ~800-1400 rpm tool rotation. Tool traversing speed of ~50 mm/min remained same for all joints. Effect of thermal state and deformation on texture and microstructure at weld nugget was investigated. Weld nugget consisted of ferrite + bainite/Widmanstatten ferrite with different matrix grain sizes depending on peak temperature. A texture around (? ₂ = 0°, φ = 30°, ? ₂ = 45°) was developed at weld nugget. Grain boundary misorientation at weld nugget indicated that continuous dynamic recrystallization influenced the development of fine equiaxed grain structure. Pole figures and orientation distribution function were used to determine crystallographic texture at weld nugget and base metal. Shear texture components D₁, D₂ and F were present at weld nugget. D₁ shear texture was more prominent among all. Large number of high-angle grain boundaries (~60-70%) was observed at weld nugget and was the resultant of accumulation of high amount of dislocation, followed by subgrain formation.     

Correlation Between the Pitting Potential Evolution and $$\varvec\sigma$$ Phase Precipitation Kinetics in the 2205 Duplex Stainless Steel

The aim of this work is to correlate the pitting potential (E_pit) evolution with the kinetics of σ phase precipitation in the 2205 duplex stainless steel aged at 850 °C after solution treatment at 1150 °C. The potentiodynamic polarization curves indicate a reduction of the pitting corrosion resistance with the aging time, which is revealed by a decrease in the E_pit values from 0.65 to 0.40 V_SCE. Thus, E_pit values are used to determine the kinetics parameters of the σ phase precipitation. The experimental transformed fraction agrees well with the one calculated by using the modified Kolmogorov–Johnson–Mehl–Avrami equation with an impingement parameter c?=?0.6.     

Evolution of grain–subgrain structure and carbide subsystem upon annealing of a low-carbon low-alloy steel subjected to high-pressure torsion

The effect of annealing on the evolution of an ultrafine-grain structure and carbides in a 06MBF steel (Fe–0.1Mo–0.6Mn–0.8Cr–0.2Ni–0.3Si–0.2Cu–0.1V–0.03Ti–0.06Nb–0.09C, wt %) has been studied. The grain–subgrain structure (d = 102 ± 55 nm) formed by high-pressure torsion and stabilized by dispersed (MC, M₃C, d = 3–4 nm) and relatively coarse carbides (M₃C, d = 15–20 nm) is stable up to a temperature of 500°C (1 h) (d = 112 ± 64 nm). Annealing at a temperature of 500°C is accompanied by the formation in regions with a subgrain structure of recrystallized grains, the size of which is close to the size of subgrains formed by high-pressure torsion. The average size and distribution of dispersed particles change weakly. The precipitation hardening and the increase in the fraction of high-angle boundaries in the structure cause an increase in the values of the microhardness to 6.4 ± 0.2 GPa after annealing at 500°C as compared to the deformed state (6.0 ± 0.1 GPa). After 1-h annealing at 600 and 700°C, the microcrystal size (d = 390 ± 270 nm and 1.7 ± 0.7 μm, respectively) increases; the coarse M₃C (≈ 50 nm) and dispersed carbides grow by 5 and 8 nm, respectively. The value of the activation energy for grain growth Q = 516 ± 31 kJ/mol upon annealing of the ultrafine-grained steel 06MBF produced by high-pressure torsion exceeds the values determined in the 06MBF steel with a submicrocrystalline structure formed by equal-channel angular pressing and in the nanocrystalline α iron.     

Softening characteristics in ultra-narrow gap GMA welded joints of ultra-fine grained steel

Abstract

When a 800 MPa grade ultra-fine grained steel with ferrite grains less than 1 μm and dispersed fine cementite is welded, fine ferrite grains are coarsened resulting in remarkable softening in the heat affected zone (HAZ). The peak temperature at an arbitrary location in HAZ during welding was calculated by heat conduction analysis and the effect of welding thermal history on the microstructure of the UFG steel HAZ was examined by microscopic observation. Softening as a result of ferrite grain coarsening was observed in the region where the peak temperature reaches between 920 and 1300 K for the ultra-fine grained steel with an Ac ₁ temperature of 980 K and Ac ₃ of 1150 K. The formation of martensite–austenite constituents started as a second phase above the Ac ₁ temperature and they curbed HAZ softening in the peak temperature range between 1000 and 1250 K.     

Effect of Al,Si, and Cr on the thermal stability and high-temperature oxidation resistance of coatings based on titanium boronitride

The thermal stability and resistance to high-temperature oxidation of multicomponent nanostructured coatings in the Ti-X-B-N (X = Al, Si, Cr) system have been studied using X-ray diffraction analysis, X-ray photoelectron spectroscopy, secondary-ion mass spectroscopy, and transmission electron microscopy. The hardness, elastic modulus, elastic recovery, friction coefficient, wear rate, and adhesion strength of the coatings have been determined. It has been established that Ti-B-N and Ti-Cr-B-N coatings exhibit a stable nanostructure and high stable mechanical and tribological properties up to 1000°C. The coatings with an fcc structure can be also employed as barrier layers preventing diffusion of metal atoms from the substrate. It has been shown that the high resistance to high-temperature oxidation of Ti-Cr-B-N and Ti-Al-Si-B-N coatings is connected with the fact that protective oxide layers based on (Ti,Cr)BO₃ and Ti _x Al _y SiO _z are formed on their surface.     

Crystallography of in-situ transformations of the <Emphasis Type="Italic">M</Emphasis><Subscript>7</Subscript>C<Subscript>3</Subscript> carbide in the cast Fe–Cr–Ni alloy

In the process of holding of the cast heat-resistant Fe–Cr–Ni (0.45C–25Cr–35Ni) alloy at 1150°C, the eutectic chromium carbide present in its structure undergoes a gradual transition M ₇C₃ → M ₂₃C₆. The gradual formation of domains of the M ₂₃C₆ carbide inside the particles of the M ₇C₃ carbide makes it possible to assume that the observed phase transition is the well-known carbide transformation of the in situ type. The mechanism of the in situ transformation of the crystal structure of the carbide from M ₇C₃ into M ₂₃C₆ with a change in the number of nearest metal neighbors of carbon atoms is explained within the previously developed combinatory model of polymorphic transitions in the metals.     

Si,Mg对铝/钢熔钎焊焊接接头力学性能的影响

采用脉冲旁路耦合电弧MIG熔钎焊方法,分别采用4043,5356铝合金焊丝对5052铝合金/镀锌钢异种金属进行了搭接焊.通过扫描电镜（SEM）,能谱仪（EDS）,X射线衍射仪（XRD）对铝/钢连接界面、接头断裂行为及断口形貌进行了分析,发现5356铝合金焊丝焊接接头的润湿角要大于4043铝合金焊丝焊接接头的润湿角,合金元素Si既可改变界面反应层金属间化合物的形态同时还可显著减少Fe₂Al₅层的厚度.拉伸试验发现5356铝合金焊丝焊接所得接头主要断裂于界面反应层,属于脆性断裂;4043铝合金焊丝焊接所得接头主要断裂于熔合区,是以韧性断裂为主的混合断裂.通过对4043铝合金焊丝焊接所得接头进行显微硬度测试,发现热影响区组织的显微硬度明显低于其它区域的显微硬度,这导致4043铝合金焊丝焊接接头主要断裂于熔合区.     

Study on gas metal arc welding procedure of stainless steel

Summary

This paper describes an investigation of the creep rupture properties of welded joints produced from W-containing 9Cr-Mo-W steel. The creep rupture properties of the HAZ are also studied using simulated HAZ specimens subjected to PWHT (post-weld heat treatment). The effect of W on the creep rupture strength of the welded joints is examined.

Creep rupture tests of GTA (TIG) welded joints are conducted. The longest creep rupture time is around 20 000 hours. In the creep rupture tests, the welded joints rupture in the base metal at higher applied stresses, rupturing in the low-ductility fine-grained HAZ adjacent to the base metal at lower applied stresses.

When the welded joints rupture in the base metal, their creep rupture strength is as high as that of the base metal. When the welded joints rupture in the HAZ, however, their creep rupture strength is lower than that of the base metal. The cracking which occurs in the HAZ is TYPE IV cracking which tends to affect the welded joints of ferritic heat-resistant steel. TYPE IV cracking is the type which occurs in the fine-grained HAZ at a lower stress than the creep rupture strength of the base metal without being associated with any heavy deformation.

In the creep rupture tests, the simulated HAZ specimens heated to a temperature around Ac1 and Ac3 give a lower creep rupture strength than that of the base metal. The simulated HAZ specimens heated to the Ac3 temperature give the lowest creep rupture strength.

A comparison of the creep rupture strengths of welded joints produced from 9Cr-Mo-W steel and 9Cr-1 Mo-Nb-V (mod. 9Cr-Mo) steel suggests that W improves the creep rupture strength of both welded joints and base metal.     

Specific Features of the Frequency Variations in Magnetic Losses in Fe–3%Si {110} Crystals in Rotating Magnetic Fields

The frequency dependence of the total and eddy-current magnetic losses P_tot and P_edd for rotary magnetization reversal has been studied on an Fe–3%Si single crystal with a thickness of d = 0.10 mm within a range of frequencies f = 20–200 Hz and induction amplitudes B_m = 0.25–1.90 T. Some new specific features in the behavior of magnetic losses have been revealed. In particular, the induction that corresponds to the maximal magnetic losses in the curve P_tot = P(B_m) has been established to monotonically decrease with increasing frequency of magnetic field rotation. The specific features of the frequency variation of magnetic losses were discussed based on the observed dynamics for the domain structure of the specimen.     

Effect of minor change in composition on toughness of weldmetal for repair of turbine blades made of martensitic stainless steel

Abstract

Repair welding procedure for cracked turbine blades, made of 13Cr–2˙6Ni–1˙1Mo martensitic stainless steel, has been developed using gas tungsten arc welding process and a twin wire filler metal. The twin wire consists of a 1˙5 mm diameter ER 16-8-2 and a 2˙0 mm diameter ER 410 filler wires tack welded along the length of the two filler wires. A two stage post-weld heat treatment at 675°C for 2 h and 615°C for 4 h, such that the first heat treatments is above the Ac ₁ temperatures of the weld metal and the second is just below its Ac ₁ temperature; has been found to be suitable for obtaining good mechanical properties for the weldment. The weldment has a good combination of transverse weldment strength and weldmetal toughness, with its room temperature yield strength and Charpy V notch impact toughness being similar to that of the turbine blade material.     

Effect of Titanium Additions on the Thermophysical Properties of Glass-Forming Cu<Subscript>50</Subscript>Zr<Subscript>50</Subscript> Alloy

Differential scanning calorimetry, laser flash method, and dilatometry were used to study the thermophysical properties of quenched Cu₅₀Zr_50–xTi_x (x = 0, 2, 4, 6, 8) alloys in the temperature range from room temperature to 1100 K. Data obtained on the heat capacity, thermal diffusivity, and density have been used to calculate the coefficient of thermal conductivity. Temperatures corresponding to the stability of martensite CuZr phase, its eutectoid decomposition, and formation in Cu₅₀Zr_50–xTi_x alloys with different Ti contents upon heating have been determined. It has been found that the thermal diffusivity and thermal conductivity of the studied alloys are low and a typical of metallic systems. As the titanium content increases, the coefficients of thermal conductivity and thermal diffusivity vary slightly. It has been shown that the low values of thermophysical characteristics correspond to the better capability of amorphization and can be a criterion for the glass-forming ability of Cu–Zr-based alloys.