Simulated Heat-affected Zone Research Articles

Effect of LCF on behavior and microstructure of microalloyed HSLA steel and its simulated CGHAZ

Numerous structural components made of high strength low-alloyed steels are often exposed to low-cycle fatigue loading during their exploitation life. In order to study the fatigue behavior of one grade of structural steels produced in Yugoslavia during the eighties of the last century, a push-pull constant amplitude strain-controlled fatigue tests were conducted at room temperatures. This paper analyses behavior of two different materials, parent steel (PM) and its coarse-grained (CG) simulated heat-affected zone (SHAZ) exposed to the action of low-cycle fatigue (LCF) loading performed in the range from 1 to 104 cycles. LCF tests were conducted at strain ratio R = -1 at strain amplitude range from 0.3% to 0.8%. The materials' Coffin-Manson parameters appearing in the strain-life and cyclic stress-strain curves were calculated. Fracture surface analysis of both materials was done. The results showed that plasticity under dynamic load conditions is satisfactory for both materials which give them good resistance to crack initiation and propagation.

Characterization of Microstructures and Fatigue Properties for Dual-Phase Pipeline Steels by Gleeble Simulation of Heat-Affected Zone.

To increase transmission efficiency and reduce operation cost, dual-phase (DP) steels have been considered for pipeline applications. Welding has to be involved in such applications, which would cause a localized alteration of materials and cause many potential fatigue issues to arise under cyclic loading. In this work, the fatigue crack propagation and fatigue life of simulated heat-affected zone (HAZ) were examined. Results indicate that when the maximum stress is at the same magnitude, the fatigue life at a peak temperature of 1050 °C is very close to that of a peak temperature of 850 °C, and both of them are higher than that of a peak temperature of 1350 °C. The changes in da/dN with ΔK for HAZ subregions are attributed to the variation of crack path and fracture mode during the crack propagation. The fatigue cracks may propagate along the bainite lath preferentially in coarse-grained HAZ (CGHAZ), and the prior austenite grain boundaries can change the crack growth direction. A considerable amount of highly misoriented grain boundaries in fine-grained HAZ (FGHAZ) and intercritical-grained HAZ (ICHAZ) increase the crack growth resistance. The difference of fatigue crack propagation behavior in HAZ subregions between actual and simulated welded joints was also discussed.

Influence of different heat-affected zone microstructures on the stress corrosion behavior and mechanism of high-strength low-alloy steel in a sulfurated marine atmosphere

The stress corrosion cracking (SCC) behavior and mechanism of the simulated heat-affected zone (HAZ) of high-strength low-alloy (HSLA) steel in a sulfurated marine atmosphere were surveyed in detail using electrochemical measurements and slow strain rate tensile (SSRT) tests combined with microstructure analysis. The SCC of the simulated HAZs is controlled by both anodic dissolution (AD) and hydrogen embrittlement (HE), which are attributed to the synergistic effect of Cl− and SO42−, as Cl−-induced localized dissolution causes microcrack initiation, and SO42--catalyzed acid regeneration facilitates microcrack propagation. The intercritical HAZ and fine-grained HAZ present high crack numbers because of the high amount of prior austenite grain boundaries (PAGBs), lath bainite boundaries (LBBs), and martensite/austenite (M/A) constituents, which act as preferential sites for hydrogen trapping and crack initiation. However, coarse-grained HAZ exhibits the highest SCC susceptibility because of the coarse PAGBs, wide lath bainites (LBs), and high local dislocation density, which promote crack propagation.

Analysis of heat-affected zone microstructures of steel P92 after welding and after post-weld heat treatment

Modern thermal power plant operating on fossil fuels has to be highly efficient and should emit low levels of CO2. One of possible ways to fulfil these requirements is to increase thermal power plant peak service temperature. P92 steel enables service temperature of up to 650 °C. It is stronger, more resistant to creep and enables higher service temperature than P91. During welding of steam boiler components made of P92, it is important to consider proper preheating, welding and post-weld heat treatment parameters in order to avoid cracks and diminish sensitivity to creep in service. The research focused on analysis of simulated heat-affected zone (HAZ) microstructures and distribution of carbides. The FG HAZ (fine-grain heat-affected zone) (Tmax is 950 °C) is the most problematic when it comes to occurrence of type IV cracks, as in this zone, the average representation of carbides on PAGB is 72%, while the average representation of carbides on PAGB in the P21 steel is 58%. After the heat treatment, maximum hardness is recorded in the CG HAZ (coarse-grain heat-affected Zone) (Tmax is 1300 °C), where the values reached 248 HV, which was below the maximum allowable value of 250 HV. Other areas of HAZ expressed the values from 218 to 227 HV. Charpy impact test at service temperatures of 650 °C showed that the CG HAZ was the most critical immediately after welding as the energy for crack initiation was 30 J. However, after obligatory post-weld heat treatment, FG HAZ appeared to be more problematic than CG HAZ, so this research showed that the crack initiation energy value increased due to the fine-grain microstructure, yet the crack propagation energy reduced.

Mechanical properties and microstructural characterization of simulated heat-affected zones in 10 wt pct Ni steel

The effects of welding thermal cycles on the mechanical properties of a high strength, high ballistic resistance 10 wt pct Ni steel were studied. Welding simulations were performed using a Gleeble 3500 thermal-mechanical simulator to replicate microstructures observed in the heat-affected zone of this steel, which is a transformation induced plasticity or TRIP steel. The microstructural influences on austenite content, strength, and toughness were determined using a variety of characterization techniques including XRD, SEM, EBSD, and STEM/EDS. The results demonstrate that with increasing peak temperature of the welding thermal cycle, the amount of austenite present in the microstructure decreases. However, the toughness results do not directly correlate with the austenite contents. Poor toughness is observed in the intercritical heat-affected zone as a result of brittle high carbon martensite in this region. This region represents the greatest challenge in terms of maintaining high ballistic resistance of welds of 10 wt pct Ni steel. These results are significant in that the toughness of this steel is not solely based on the austenite content, which would be expected given it is a TRIP steel, but instead is a function of other microstructural influences.

HAZ characterization of welded 42SiCr steel treated by quenching and partitioning

An experimental steel 42SiCr was heat treated with a quenching and partitioning treatment in order to achieve high strength and at the same time acceptable ductility. This crucial combination requires the formation of a martensitic microstructure with a small volume fraction of retained austenite embedded on a microscopic scale. The objective of this work was to investigate the evolution of microstructure and hardness in the heat affected zone (HAZ) of welds in this 42SiCr and its major influence factors. Thermo-physical simulations with a dilatometer yielded samples of each zone in the HAZ and allowed a detailed investigation of HAZ microstructures and properties. A decrease in hardness from 580 HV in the quenched and partitioned base material to the HAZ, with minimum hardness values of 370 HV in a region exposed to temperatures close to the Ac3 temperature, was revealed. Simulation showed that the experienced peak temperature is predominantly determining the HAZ properties. The microstructure after quenching and partitioning treatment consisted of fine martensite with presumed retained austenite. The HAZ regions that were heated above Ac3 temperature can be characterized to have a fine bainitic/martensitic microstructure, while the regions heated to more than 1100 °C show coarser grain, with bainitic/martensitic features.To support the HAZ simulation results sheets of this steel were rolled and resistance spot welds were produced. The welds were characterised with respect to the microstructure in the heat affected zone, as well as the mechanical properties of the joint. Results proved that the data obtained by thermo-physical simulation is in very good agreement with the spot welding results. On a macroscopic scale the welding results indicate very good joint formation but very brittle fracture behaviour using a conventional resistance spot welding process for the quenching and partitioning steel. However, despite the relatively high carbon equivalent of the material, also ductile aspects of fracture were observed.

Constitutive properties and plastic instabilities in the heat-affected zones of advanced high-strength steel spot welds

Recent publications have shown that the load-bearing capability of advanced high-strength steel (AHSS) spot welds do not scale linearly with the tensile strength of the base metal. Although this unexpected degradation of welds has been linked to the tempering of the martensite phase in the heat-affected zone (HAZ), the detailed elastic–plastic stress–strain behavior in different regions of the HAZ has not been reported. In this research, the plastic flow behavior in samples subjected to simulative weld thermal cycles with peak temperatures (TPeak) ranging from 350 to 1250 °C was evaluated. Results from this study showed that the microstructural changes in the HAZ could result in large mechanical heterogeneity in AHSS spot welds. In addition, plastic instability in the form of yield point phenomena was observed. The yield point phenomena in these simulated HAZ samples, in contrast to base metal samples, were confirmed by the observation of Luders band and stress fluctuations before the onset of strain hardening using digital image correlation. The local plastic instability was correlated to the chemical composition of the steel, initial microstructure, and thermal cycle. The Luders front velocity maps revealed heterogeneous nucleation and growth of multiple Luders bands with different front speeds. Interestingly, inter-critical (Ae1 < TPeak < Ae3) HAZ samples show minimum yield strength even though the maximum softening occurs in the subcritical HAZ (TPeak < Ae1) samples.

Prediction of Creep Rupture Time Using Constitutive Laws and Damage Rules in 9Cr–1Mo–V–Nb Steel Welds

Long-term creep tests for the parent metal, a simulated heat-affected zone (HAZ) and a weld joint were conducted based on 9Cr–1Mo–V–Nb steel. The creep rupture time, tr, for the weld joint was predicted by computationally simulating creep damage based on Norton’s law combined with the time exhaustion rule (TER), where the equivalent stress (σeq), maximum principal stress (σ1), or Huddleston stress (σhud) was used to evaluate the rupture time. Creep damage analysis was also conducted based on the Hayhurst-type damage mechanics rule (HDR), in which creep rupture time was evaluated in relation to the rupture stress, σr. The computed creep rupture times and damage distributions were compared with the experimentally obtained rupture time and void distribution of the actual weld joint, respectively. Furthermore, the effect of the bevel angle of the HAZ was examined. The key findings from this study were as follows: (1) cautious predictions could be obtained for a loading stress of 80 MPa and higher; (2) the computed fracture initiation positions were on the HAZ boundaries, consistent with the type IV fracture observed at the stress of 80 MPa; (3) the magnitudes of the predicted rupture times were tr (σ1) < tr (σhud) ≈ tr (σr) < tr (σeq); (4) the bevel angle dependence previously reported was reasonably reproduced with the TER models that used σeq and σhud and with the HDR model.

Creep Property of Boron Added 9Cr Heat Resistant Steels after Welding

Although welding results in premature failure by type IV fracture under high temperature creep conditions, the alloy design of light elements such as boron addition and nitrogen reduction enhances the creep lifetime of 9Cr heat resistant steel. In particular, the simulated heat affected zone (SHAZ) sample of new 9Cr steel (called TA steel) shows about 10 times longer creep lifetime than that of the standard Gr. 91 steel. The welded TA steel is thus expected to exhibit good creep properties because its SHAZ sample has coarser grains and suppresses type IV fracture. The preservation of base metal’s microstructure after welding results from the precipitate morphology, such as high grain boundary coverage by precipitates and low amount of MX being nucleation sites of ferrite grains during the a-g phase transformation. In addition, the increase of stability of M23C6 affects high pinning pressure toward grain boundary migration upon rapid heating during welding. First-principles calculations confirm the increased stability when boron is absorbed by M23C6. Moreover, the calculations reveals that boron decreases the coherency between matrix and M23C6, suppressing grain coarsening during creep tests in TA steel. It is concluded that the increased microstructural stability during welding and long high temperature exposure generates the elongated creep lifetime in welded TA steel including about 0.01 wt% boron and less than 0.01 wt% nitrogen.

The Microstructure and Pitting Resistance of 2002 Lean Duplex Stainless Steel after the Simulated Welding Thermal Cycle Process.

In this paper, thermal cycles with different heat inputs and cooling rates were investigated for a novel lean duplex stainless steel 2002 using a welding simulation. The microstructure and pitting resistance of the simulated heat-affected zones were studied. With the increasing heat input, the amount and size of the austenite phase both increased, along with a transformation from rods to dendritic structures. The critical pitting temperature (CPT) and the pitting potential (Epit) both increased first and then declined as the heat input increased, indicating a strong dependence of pitting resistance on the heat input. For the different cooling rates, the amount of ferrite increased as the cooling rate increased from 0.25 °C/s to 20 °C/s. The CPT and Epit both increased with the increasing cooling rates, indicating an improved pitting resistance. The pits initiated preferentially at the boundaries of ferrite and austenite due to the precipitation of M23C6 in the specimens with different cooling rates.

Characterizing Creep Behaviour of Modified 9Cr1Mo Steel by using Small Punch Impression Technique for Thermal Powerplants

Modified 9Cr1Mo (P91) having ferritic/martensitic structure is a candidate material for thermal powerplant industry due to its enhanced creep resistance. Fusion welding of P91 steel nurtures heat affected zone (HAZ) which gives rise to infamous type IV failure/cracking. The origination of type IV cracking still remains ambiguous mainly due to sampling constraints from P91 - HAZ subzones owing to thin width of HAZ. In present work, this issue is investigated with the help of small punch impression creep technique. Thermo – mechanical simulator Gleeble®-3800 is used to physically simulate HAZ subzones of P91 steel representing HAZ – CG, HAZ – FG and HAZ – IC. Impression creep tests done on parent metal P91 and its simulated HAZ subzones at a stress level of 270 MPa and 600℃ reveal HAZ – IC as the weakest subzone followed by HAZ – FG. Coarsening of chromium rich carbides on prior austenite grain boundaries (PAGBs) associated with blended phase transformations in HAZ – IC render it as the most degraded HAZ subzone in P91 steel thus becoming an initiating location for type IV cracking. Furthermore, presence of relatively fine carbides combined with less inter-lath spacing in P91 steel and its HAZ – CG contribute towards retarding creep rate leading to an increase in heterogeneous creep behaviour among HAZ subzones and parent metal.

Effect of inclusions on toughness and microstructures in simulated coarse-grain heat-affected zones of Al/Ti deoxidised and Nb/V microalloyed steels

ABSTRACTTo explore the effect of inclusions in the coarse-grain heat-affected zone (CGHAZ), four groups were designed using the ‘Al/Ti deoxidised + Nb/V microalloyed’ technical route. The toughness of the CGHAZ of Al–Ti–V–N steel was greatly promoted by the presence of polygonal ferrite (PF) after high heat input rather than the traditionally used acicular ferrite (AF). Furthermore, micron-sized inclusions in AF or PF were analysed, and the results indicated that the TiN and V (CN) particles attached on the Ti/Al oxide inclusions induced the nucleation of AF and PF. That can be concluded that the nucleation mechanism of PF is similar to that of AF. In addition, smaller micron- and nanoscale-size inclusions observed by transmission electron microscopy could not induce the nucleation of AF and PF because of the low heterogeneous nucleation energy. Nevertheless, these inclusions can have a drag effect on grain boundary migration.

Microstructure Evolution and Toughness Variation of Simulation HAZ with Large Heat Input Welding for E40 Ship Plate Steel

This Charpy impact toughness and microstructure evolution of simulated heat affected zone (HAZ) for E40 ship plate steel have been investigated in this experiment. Simulation of large heat input welding was conducted by thermal simulator to research microstructure evolution of HAZ at 1400°C peak temperature. Results show that microstructure at HAZ was mainly composed of high intensity of acicular ferrite (AF) and intragranular ferrite (IGF), and grain boundaries ferrite (GBF). Hardness of HAZ decreases with heat input (HI) increasing. Impact toughness for steel specimens with HI of 500 kJ/cm was 141 J. TiN precipitates nucleation and growth at HAZ was analyzed by TEM, indicating that TiN size increases and the number of TiN decreases at HAZ with HI increasing. The reason is that the specimens with HI of 400 kJ/cm has longer cooling time from 800°C to 500°C than 120 kJ/cm, which is favorable to growth of TiN particle at HAZ.

Effect of Ti–Mg–Ca treatment on properties of heat-affected zone after high heat input welding

The combined influence of Mg and Ca treatment on the properties of heat-affected zone (HAZ) of low-carbon steel after high heat input welding was systematically studied. Experimental steels deoxidized with different elements were prepared, i.e., C–Mn steel with Al, Ti–Ca steel with Ti and Ca, Ti–Mg–Ca steel with Ti, Mg and Ca. Results showed that the inclusions in C–Mn steel were mainly Al2O3 and MnS with low density and large size. However, the average size was refined to only ~ 0.34 μm in Ti–Mg–Ca steel and the amount increased remarkably. Microstructure of simulated HAZ for 200 kJ/cm changed from ferrite side plates or upper bainite to acicular ferrite after treatment with Ti, Mg and Ca. Ca addition decreased the strain field around inclusions and enhanced the ability of acicular ferrite nucleation. In situ observation of Ti–Mg–Ca steel showed that the movement of austenite grain boundaries was retarded and nucleation sites of acicular ferrite were greater than Ti–Ca steel because of Mg addition. Impact energy of HAZ at − 40 °C was increased from 7 to 232 J and showed excellent stability because of Ti–Mg–Ca treatment. High volume fraction of acicular ferrite acted as obstacles toward cleavage cracks.

Quantitative analysis of microstructure and impact toughness in the simulated coarse-grained heat-affected zone of Cu-bearing steels

ABSTRACTBecause of 0.32% Cu addition, the inclusion is modified from Al-Ti oxide covered with layer of MnS in Cu-free steel to Al-Ti oxide with shell of (Mn,Cu)S in Cu-bearing steel. The quantitative result reveals that the frequency of inclusion effectively for acicular ferrite nucleation, fraction of acicular ferrite and high angle grain boundary in the simulated coarse-grained heat-affected zone (CGHAZ) of Cu-bearing steel are higher than that of Cu-free steel, respectively. Therefore, the high density of acicular ferrite microstructures in CGHAZ of Cu-bearing steel with smaller crystallographic grain size has superior impact toughness than that in Cu-free steel.

Investigation of Hydrogen Embrittlement Susceptibility of X80 Weld Joints by Thermal Simulation

The objective of this study was to investigate the hydrogen embrittlement (HE) susceptibility and influence mechanism of X80 weld joints. Slow strain rate testing (SSRT) under in situ H-charging, combined with microstructure and fracture analysis, was performed on the base metal (BM), weld metal (WM), thermally simulated fine-grained heat-affected zone (FGHAZ) and coarse-grained heat-affected zone (CGHAZ). Results showed that the WM and simulated HAZ had a greater degree of high local strain distribution than the BM; compared to the CGHAZ, the FGHAZ had lower microhardness and more uniformly distributed stress. SSRT results showed that the weld joint was highly sensitive to HE; the HE index decreased in the following sequence: FGHAZ, WM, CGHAZ and BM. The effect of the microstructure on HE was mainly reflected in microstructure, local stress distribution and microhardness.

Microstructural Characteristics and m23c6 Precipitate Behavior of the Course-Grained Heat-Affected Zone of T23 Steel without Post-Weld Heat Treatment

The microstructural characteristics of a simulated heat-affected zone (HAZ) in SA213-T23 (2.25Cr-1.6W steel) used for boiler tubes employed in thermal power plants were investigated using nital, alkaline sodium picrate, and Murakami’s etchants. In order to investigate the microstructure formation process of the HAZ in the welding process, simulated HAZ specimens were fabricated at intervals of 100 °C for peak temperatures between 950 and 1350 °C, and the microstructural features and precipitate behavior at various peak temperatures were observed. The alkaline-sodium-picrate-etched microstructures exhibited a black dot or band, which was not observed in the natal-etched microstructure. As the temperature increased from 950 to 1350 °C, the black dot and band became wider and thicker. Experimental analyses using an electron probe micro-analyzer, electron backscatter diffraction, and transmission electron microscopy revealed the appearance of austenite in the black dot region at a peak temperature of 950 °C; its amount increased up to a peak temperature of 1050 °C and thereafter decreased as the peak temperature further increased. The amount of M23C6 decreased with an increase in peak temperature. Based on these results, we investigated the behaviors of austenite and M23C6 as functions of the peak temperature.

Evolution of microstructure and microhardness of the weld simulated heat-affected zone of Ti-22Al-25Nb (at.%) alloy with continuous cooling rate

The influences of continuous cooling rate on microstructure and microhardness of the weld simulated heat-affected zone (HAZ) of Ti-22Al-25Nb (at.%) alloy were investigated in this study. The continuous cooling process of the weld HAZ of Ti-22Al-25Nb alloy was simulated using a Gleeble3500 thermo-mechanical simulator. The phase constituent investigated by X-ray diffraction showed that the weld simulated HAZs were all composed of lath-shaped O phase, equiaxed α2 particle and β/B2 matrix at different continuous cooling rates. Microstructural observations by scanning electron microscopy indicated that the size of O phase significantly decreased and the shape of α2 particle did not obviously change with the increase of continuous cooling rate. Quantitative analysis presented that with increasing continuous cooling rate the volume fraction of O phase decreased, while that of α2 and β/B2 phases increased. Phase transformation temperatures were determined by using dilatometric analysis and the weld simulated HAZ continuous cooling transformation diagram was determined. The microhardness of the weld simulated HAZ increased with the increase of continuous cooling rate.

Cyclic plasticity and low cycle fatigue damage characterisation of thermally simulated X100Q heat affected zone

This paper presents the cyclic plasticity and low cycle fatigue (LCF) damage characterisation of thermally simulated heat affected zone (HAZ) for API 5L X100Q weldments. Microstructures representative of the HAZ for two cooling rates are generated using a Gleeble thermomechanical simulator for manufacture of strain-controlled cyclic plasticity test specimens. The simulated HAZ specimens are subjected to a strain controlled test programme which examines the cyclic effects of strain-range and the tensile response at room temperature. A modified version of the Chaboche rate independent plasticity model, which accounts for early stage damage is implemented to characterise the cyclic plasticity response, including isotropic and kinematic hardening effects. The constitutive parameters are fitted to experimental data using an optimisation procedure developed within a MATLAB code. The measured response of the simulated HAZ specimens is compared to that of the X100Q parent material (PM), and the simulated HAZ is shown to share the early stage fatigue damage behaviour of the PM, but exhibits significantly a higher yield and cyclic strength.

Effects of Niobium on the Mechanical Properties and Corrosion Behavior of Simulated Weld HAZ of HSLA Steel

Simulated microstructures of the TZ, ICHAZ, FGHAZ, and CGHAZ of weld joints made from two kinds of HSLA steels with 0 or 0.079 wt pct Nb were prepared by means of heat treatment. Optical microscopy and transmission electron microscopy were used to observe microstructures and the distribution of nanosized precipitates in the simulated weld heat-affected zone (HAZ). Mechanical properties of the simulated HAZ were measured by tensile tests, and the corrosion behavior in simulated seawater was studied using electrochemical and immersion tests. It was shown that the ICHAZ and CGHAZ possess the worst overall mechanical properties in both kinds of HSLA steels, and the corrosion resistance in the descending order was the BM, TZ, FGHAZ, ICHAZ, and CGHAZ. Contrasting Nb-bearing and Nb-free steel demonstrated that the strength and corrosion resistance of the simulated HAZ were enhanced by Nb microalloying, which resulted in precipitation, homogeneous microstructures, and other relative sequences. Moreover, the surface of the Nb-bearing steel formed compact corrosion product films with higher resistance to ion migration; thus, the initiation and propagation of pitting holes were effectively inhibited.