Fine structure in the inter-critical heat-affected zone of HQ130 super-high strength steel

The microstructure in the intercritical heat-affected zone (ICHAZ) of HQ130 steel, has been investigated by thermo-simulation test, SEM and TEM. The problem of toughness decrease in the ICHAZ (T _p = 800°C) as well as the effect of M-A constituent and carbide precipitation on brittleness was analysed. The test results indicated that the microstructure in the ICHAZ of HQ130 steel was mostly a mixture of lath martensite (ML) and granular bainite (Bg) with a fine but nonuniform grain structure. The cause of brittleness in the ICHAZ was related to production of the M-A constituent in the local region and carbide precipitation. By controlling the welding heat input carbide precipitation and the formation of the M-A constituent can be avoided or decreased.     

Microstructure characterization in the weld metals of HQ130 + QJ63 high strength steels

Microstructural characterization of the weld metals of HQ130 + QJ63 high strength steels, welded under 80% Ar + 20% CO₂ gas shielded metal arc welding and different weld heat inputs, was carried out by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The relative contents of acicular ferrite (AF) and pro-eutectic ferrites (PF) in the weld metals were evaluated by means of XQF-2000 micro-image analyser. The experimental results indicate that there is acicular ferrite in the grain and some pro-eutectic ferrite on the boundary of original austenite grains when the weld heat input is small (E = 9.6 kJ/cm), but the main microstructure is ferrite side plate (FSP) when the heat input is larger (E = 22.3 kJ/cm). So the weld heat input should be strictly controlled in the range 10 ∼ 20 kJ/cm and then the content of pro-eutectic ferrite is limited to < 25%. Thus weld metals of HQ130 + QJ63 high strength steels with high toughness and excellent resistance to cracking can be ensured.     

Finite element analysis of residual stress in the welded zone of a high strength steel

The distribution of the residual stress in the weld joint of HQ130 grade high strength steel was investigated by means of finite element method (FEM) using ANSYS software. Welding was carried out using gas shielded arc welding with a heat input of 16 kJ/cm. The FEM analysis on the weld joint reveals that there is a stress gradient around the fusion zone of weld joint. The instantaneous residual stress on the weld surface goes up to 800 ∼ 1000 MPa and it is 500 ∼ 600 MPa, below the weld. The stress gradient near the fusion zone is higher than any other location in the surrounding area. This is attributed as one of the significant reasons for the development of cold cracks at the fusion zone in the high strength steel. In order to avoid such welding cracks, the thermal stress in the weld joint has to be minimized by controlling the weld heat input.     

Effect of welding heat input on microstructure and mechanical properties of simulated HAZ in Cu containing microalloyed steel

The influence of weld thermal simulation on ICGC HAZ microstructure and mechanical properties of Cu containing Nb-Ti-microalloyed steel has been investigated. Low heat input of 0.7 kJ/mm (simulated fast cooling of Δt _8/5 = 5 s) and high heat input of 4.5 kJ/mm (simulated slow cooling of Δt _8/5 = 61 s) were used to generate double-pass thermal cycles with peak temperatures of 1350 and 800 °C, respectively. The microstructure after high heat input mainly consisted of polygonal and quasi-polygonal ferrite (QF) grains with certain amount of acicular ferrite, whereas, after the low heat input, microstructure mainly consisted of lath or elongated bainite–ferrite, QF and M–A constituents. The size of ferrite grains decreased and volume of M/A constituents increased with fast cooling rate. The precipitation characteristics were found to be similar in both cooling rates. However, the precipitation of Cu-related phases was promoted by slow cooling rate. By fast cooling rate, the investigated steel exhibited an increase in hardness from 187_HV to 197_HV. Consequently higher yield strength with considerable loss in the (−10 °C) CTOD fracture toughness (δ_{fast cooling} = 0.86 mm and δ_{slow cooling} = 1.12 mm) were demonstrated.     

HAZ fracture toughness of titanium containing steels

Abstract

Steels containing various combinations of microalloying elements (Nb, V, and Ti) were welded at heat inputs from 3 to 6 kJ mm^?1. It was shown by detailed crack tip opening displacement fracture toughness testing of coarse grained heat affected zone (HAZ) regions in single pass weld deposits that the poorest toughness properties were exhibited by steel containing a combination of Nb, V, and Ti. Steel microalloyed with only titanium had the best HAZ fracture toughness at all heat input levels. Detailed microstructural analysis, grain size measurement, hardness, and precipitation in HAZ regions were evaluated to explain the fracture toughness properties observed.

MST/887     

Ultrasonic Analysis of Cracking Propagation Morphology in the Fusion Zone of High Strength Steel

Cracking morphology in the fusion zone of HQ130 high strength steel was researched by “the y-slit test“and “three-point bend test“,ultrasonic test and microscope.HQ130 and Q163 high strength steel welded by Ar CO2 gas shielded arc welding under the condition without preheating.Experimental results indicated that welding cracks were produced in the partially melted zone of the weld root ofHQ130 steel side and propagated parallel to the fusion zone.The cracks were developed alternatively between the weld and the partially melted zone, and are not strictly ruptured at W/F(weld metal/fusion zone) boundary surface.Controlling weld heat input(E) about 16kJ/cm could make the cracking rate lowest and satisfy the performance requirement of welded joint zone.     

Mechanical assessment of autogenous gas tungsten arc weldments of a super alpha-2 alloy

Butt welding of a 2-mm-thick super alpha-2 alloy (Ti-23Al-9Nb-2Mo-0.9Si) was carried out employing autogenous gas tungsten arc technique. Process parameters were adjusted to achieve full-penetration weld. Only β-Ti-based phase was detected in the fusion zone; however, minor quantities of a Ti₃Al-based structure were found at heat input higher than 518 J mm⁻¹. No cracking was observed within the fusion zone and its associated heat-affected zones. Subgrain boundaries and acicular transformation products were observed within the HAZ grains. The acicular transformation products are believed to be responsible for the higher microhardness value of the HAZ compared to those of the fusion zone and base material. It was found that the fusion zone and HAZ had lower strength and ductility than the base material. Fracture occurred within the HAZ, which is attributed to its higher microhardness value and acicular transformed microstructure. The maximum weld tensile strength achieved was about 90% of that of the base material.     

Effects of acicular ferrite on charpy impact properties in heat affected zones of oxide-containing API X80 linepipe steels

This study was concerned with effects of acicular ferrite on Charpy impact properties in heat affected zones (HAZs) of two API X80 linepipe steels containing oxides. In the one steel, Mg and O₂ were additionally added to form a larger amount of oxides than the other steel, which was a conventional X80 steel containing a considerable amount of Al and Ti. Various HAZ microstructures were obtained by conducting HAZ simulation tests under different heat inputs of 35 kJ cm⁻¹ and 60 kJ cm⁻¹. Oxides present in the API X80 linepipe steels were complex oxides whose average size was 1-2 μm, and the number of oxides increased with increasing amount of Mg and O₂. The volume fraction of acicular ferrite present in the steel HAZs increased with increasing number of oxides, and decreased with increasing heat input. When the volume fraction of acicular in the HAZ was higher than 20%, Charpy impact energy at −20 °C was higher than 100 J as the ductile fracture mode was dominant. Particularly in the steel HAZs having a larger amount of oxides, Charpy impact properties were excellent because oxides worked as nucleation sites of acicular ferrite during welding. Charpy impact properties of the HAZs could be well correlated with the volume fraction of acicular ferrite and number of oxides under different heat input conditions.     

XRD and TEM analysis of microstructure in the welding zone of 9Cr-1Mo-V-Nb heat-resisting steel

Under the condition of tungsten inert gas shielded welding (TIG) + shielded metal arc welding (SMAW) technology, the microstructure in the welding zone of 9Cr-1Mo-V-Nb (P91) heat-resisting steel is studied by means of X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The test results indicate that when the weld heat input (E) of TIG is 8.5 ∼ 11.7 kJ/cm and the weld heat input of SMAW is 13.3 ∼ 210 kJ/cm, the microstructure in the weld metal is composed of austenite and a little amount of δ ferrite. The substructure of austenite is crypto-crystal martensite, which included angle. There are some spot precipitates in the martensite base. TEM analysis indicates that the fine structure in the heat-affected zone is lath martensite. There are some carbides (lattice constant, 1.064 nm) at the boundary of grain as well as inside the grain, most of which are Cr₂₃C₆ and a little amount of (Fe, Me)₂₃C₆.     

Microstructure, Impact Toughness and TEM Analysis in the CGHAZ of HQ130 High Strength Steel

1.IntroductionThedifferentpartsintheheat-affectedz0neofthehighstrengthsteelexperienceddifferentweldthermalcycles,inwhichthechangeofmicrostructureandper-formanceinthecoarsegrainheat-affectedzone(CG-HAZ)nearthefusionzoneismost0bvious,andcoldcrackingiseasytobeproducedandspreadinthiszone.CGHAZwasconsideredasone0fthem0stweakpositioninwholeweldingjoilltzone,andllu-merousinvestigatorshavepaidcl0seattentiontotheCGHAZl1'2].HQ130steel,whichhastensilestrength(UTs)ofmorethan1300MPa,isanewlydevelop…     

HAZ microstructure simulation in welding of a ultra fine grain steel

In the present work the evolution of grain structure in the weld HAZ (heat affected zone) under welding thermal cycle was simulated. Especially the grain growth in the HAZ of a SS400 ultra fine grain steel was investigated. An integrated 3-D Monte Carlo (MC) simulation system for grain growth of the weld HAZ was developed based on Microsoft Windows. The results indicate that MC simulation is an effective way to investigate the grain growth in weld HAZ. The method not only simulates the non-isothermal dynamics process of the grain growth in the weld HAZ, but also visualizes the austenite grains realistically. Moreover, the thermal pinning effect can be easily included in the simulation process. The grain sizes of the CGHAZ (coarse grain heat affected zone) obtained from MC simulation are basically in agreement with the experimental measurement of the real welded joints under different heat input. Furthermore, the simulation indicates that the grain growth degree is higher for the SS400 ultra fine grain steel compared to conventional steel. With the increase in the heat input, the grain growth of the CGHAZ rapidly increases. Because the activation energy of the grain growth is lower for the SS400 ultra fine grain steel, austenite grains can grow at a relatively lower temperature, hence the range of the CGHAZ becomes wider.     

Effect of nickel on microstructure and mechanical properties of heat affected zone of low carbon steel

Abstract

In the present study, the effect of nickel along with varying heat input on the microstructure and mechanical properties of the heat affected zone (HAZ) of a low carbon steel was investigated. Experiments were carried out in which low carbon steel specimens with five different nickel contents, 1, 2, 2.9, 4.1, and 5.2 wt-%, were welded using a submerged arc welding machine with heat inputs of 0.5, 1, and 2 kJ mm^-1. Following welding, the microstructure, hardness, and toughness of the HAZs were determined. From the results, attempts were made to establish a relationship between heat input, nickel content, microstructure, hardness, and toughness. Charpy impact testing and microstructural observation showed that, for a heat input of 0.5 kJ mm^-1, nickel contents between 2.9 and 5.2 wt-% were effective in forming lower transformation products, such as martensite, thereby producing lower toughness values. It was subsequently found that, taking into consideration the microstructure, hardness, and toughness of the HAZ, a lower heat input for a nickel content of 1 wt-% and a medium heat input for nickel contents between 2 and 5.2 wt-% gave good results.     

Microstructure and Fracture Morphology in the Welding Zone of T91 Heat—resisting Steel Used in Power Station

Microstructure performance in the welding zone of T91 heat-resistant steel under the condition of TIG welding was researched by means of metallography, X-ray diffraction and scanning electron microscope (SEM). Experimental results indicated that microstructure of T91 weld metal was austenite + a little amount of S ferrite when using TGS-9cb filler wire. Substructure inside the austenite grain was crypto-crystal lath martensite, on which some Cr23C6 blocky carbides were distributed. The maximum hardness (HRC44) in the welding zone is near the fusion zone. There existed no obvious softening zone in the heat-affected zone (HAZ). For T91 steel tube of $63 mmx5 mm, when increasing welding heat input (E) from 4.8 kJ/cm to 12 5 kJ/cm, fracture morphology in the fusion zone and the HAZ changed from dimple fracture into quasi-cleavage fracture (QC). Controlling the welding heat input of about 9.8 kJ/cm is suitable in the welding of T91 heat-resistant steel.     

Microstructure and fracture behaviour of Ti<Subscript>3</Subscript>Al/TC4 dissimilar materials joints welded by electron beam

Electron beam was used to join TC4 alloy to Ti₃Al-based alloy. The composition of the weld was analysed by XRD and TEM and the results showed that the weld mainly composed of α′ martensites. The change of heat input had little influence on the composition of the weld but can make the grain size increasing. The fracture path of the joints was mainly decided by the microstructure of the weld and started from coarse grain zone to HAZ and base metal of Ti₃Al alloy.     

Comparison of creep behaviour of 2.25Cr–1Mo/9Cr–1Mo dissimilar weld joint with its base and weld metals

Abstract

Evaluation of the creep behaviour of 2.25Cr–1Mo and 9Cr–1Mo ferritic steel base metals, 9Cr–1Mo steel weld metal, and 2.25Cr–1Mo/9Cr–1Mo ferritic–ferritic dissimilar weld joints has been carried out at 823 K in the stress range 100–260 MPa. The weld joint was fabricated by shielded metal arc welding using basic coated 9Cr–1Mo electrodes. Investigations of the microstructure and hardness variations across the joint in the as welded, post-weld heat treated (973 K/1 h), and creep tested conditions were performed. The heat affected zone (HAZ) in both the steels consisted of a coarse prior austenitic grain region, a fine prior austenitic grain region, and an intercritical structure. In the post-weld heat treated condition, a white etched soft decarburised zone in 2.25Cr–1Mo steel base metal and a black etched hard carburised zone in 9Cr–1Mo steel weld metal around the weld fusion line developed. Hardness troughs also developed in the intercritical HAZ regions of both the steels. The width of the carburised and decarburised zones and hardness differences of these zones were found to increase with creep exposure. The 9Cr–1Mo steel weld metal showed higher creep strength compared to both the base metals. The 9Cr–1Mo steel base metal exhibited better creep resistance than the 2.25Cr–1Mo steel base metal at lower applied stresses. The dissimilar joint revealed lower creep rupture strength than both the base metals and weld metal. The creep strain was found to concentrate in the decarburised zone of 2.25Cr–1Mo steel and in the intercritical HAZ regions of both the steels. Creep failure in the stress range examined occurred in the intercritical HAZ of 2.25Cr–1Mo steel even though this region showed higher hardness than the decarburised zone. Extensive creep cavitation and cracks were observed in the decarburised zone.     

An improvement in creep strength of thermo-mechanical treated modified 9Cr-1Mo steel weld joint

Modified 9Cr-1Mo steel weld joints generally experience the type IV premature failure in the intercritical region (ICR) of HAZ under long term creep exposure at high temperature. Possibility of improving the resistance of this joint to type IV cracking through thermo-mechanical treatment (TMT) of the steel has been explored. Weld joints have been fabricated from the TMT and conventional normalized and tempered (NT) steels using electron beam (EB) welding process. Creep tests have been carried out on NT and TMT steels joint at 923 K (650°C) and 110–100 MPa applied stress. Creep rupture life of the TMT weld joint was significantly higher than the NT steel weld joint. Significant variations of microstructural constituents such as M₂₃C₆ precipitate; lath structure and hardness across the joint have been examined in both the joints. The coarser M₂₃C₆ precipitate and lath, and subgrain formation in the ICR resulted in the soft zone formation and was predominant in the ICR of NT steel joint. The enhanced MX precipitation through TMT processing and reduction in coarsening of M₂₃C₆ precipitate under thermal cycle resulted in improved creep rupture strength of TMT steel weld joint.     

Effects of heat input on microstructure and mechanical properties of Fe–2Cr–Mo–0.12C steel

ABSTRACT

Thermal simulated specimens with the heat inputs of 20, 50 and 80?kJ/cm were used to investigate the effects of heat input on the microstructure and mechanical properties of the Fe–2Cr–Mo–0.12C pressure-vessel steel. The results indicated that the microstructures in the coarse-grained heat affected zone of tested steels with various heat inputs were mainly consisted of lath martensite and bainite ferrite. As the heat input increased, the fraction of martensite decreased and the bainite ferrite fraction increased. The toughness (tested at ?40°C) and hardness for the heat input of 50?kJ/cm were 102?J and 346?HV, respectively, which was attributed to the high-volume fraction (60%) of the high-angle grain-boundary and the fine bainite lath.

This paper is part of a thematic issue on Nuclear Materials.     

Mechanical properties of heat affected zone in bainitic high strength low alloy steel

Abstract

The Charpy impact energy of the heat affected zone (HAZ) in a high strength low alloy steel is investigated by thermal simulation. The Charpy impact energy value first increases and then decreases with the increasing of cooling time. It is believed that this behaviour is closely related to changes in the amounts of martensite–austenite in the microstructures. Microstructural characterisation of the HAZ by TEM shows that the optimum Charpy impact energy values are associated with a microstructure consisting of lower bainite and a fine lath martensite.     

不同热输入下F460钢焊接粗晶热影响区韧脆转变的内在机理

使用Gleeble 3800热模拟试验机模拟F460钢单道次焊接条件下焊接粗晶热影响区的热循环过程,通过光镜(OM)、扫描电镜(SEM)分析热影响区的显微组织、确定临界事件,通过ABAQUS软件计算临界解理断裂应力σf,进而系统分析不同焊接热输入E下韧脆转变温度变化的内在机理。结果表明:随着E的提高,焊接粗晶热影响区显微组织依次为少量板条马氏体和大量细密的板条贝氏体,板条贝氏体较多的板条/粒状贝氏体,粒状贝氏体较多的板条/粒状贝氏体,粗大的粒状贝氏体。原始奥氏体晶粒、贝氏体团的最大尺寸随着E的提高而变大。在完全解理断裂的冲击断口上,寻找停留在缺口尖端附近的残留裂纹,通过对比残留裂纹长度、原始奥氏体晶粒大小、贝氏体团尺寸,发现不同E下解理断裂的临界事件尺寸都是贝氏体团大小,而临界事件尺寸越小,韧脆转变温度越低。此外,通过有限元模拟缺口尖端的应力分布得到σf,σf越大冲击韧度越好,随着E的提高σf降低,故进一步说明随着E的提高韧脆转变温度Tk上升的内在机理。     

热输入对Q890高强钢焊缝组织及性能的影响

针对国内某钢厂最新研制的Q890高强钢,采用三种不同的热输入对其进行气体保护焊接,研究了不同热输入对焊缝金属组织、硬度及冲击韧性的影响.结果表明,3种热输入下,焊缝组织主要以板条贝氏体为主,并含有粒状贝氏体、少量的板条马氏体和残余奥氏体.随着热输入的增大,焊缝组织中贝氏体铁素体板条粗化,板条马氏体逐渐减少,而粒状贝氏体逐渐增多,部分残余奥氏体由薄膜状向块状转变;焊缝金属硬度随着热输入的增大而下降;焊缝金属的冲击韧性亦呈逐渐下降的趋势.