Susceptibility to Hot Cracking and Weldment Heat Treatment of Haynes 230 Superalloy

This study investigates the susceptibility of hot cracking and weldment heat treatment of Haynes 230 superalloy. The Varestriant test was conducted to evaluate this susceptibility. Welding was performed by gas tungsten arc welding （GTAW） and plasma arc welding （PAW） with stress relief heat treatment and solid solution heat treatment. A tensile test is then performed to measure the changes in the mechanical properties of the heattreated material. The results indicate that the number of thermal cycles does not affect the susceptibility of Haynes 230 superalloy to hot cracking. However, it does increase the strain. In weldment of heat treatment, stress relief annealing increases the yield strength and tensile strength of the welded parts. The section of the tensile specimens shows fibrous fractures on the welded parts, regardless of whether they are heat-treated.     

On preventing HAZ cracking in laser welded DS Rene 80 superalloy

Abstract

The heat affected zone (HAZ) cracking behaviour in a laser beam directionally solidified (DS) Rene 80 nickel based superalloy subjected to preweld heat treatments was studied. The HAZ cracks in the alloy are grain boundary liquation cracks caused by liquation reaction of both non-equilibrium secondary solidification product, MC carbides and equilibrium solid state reaction product, γ′ precipitates. In contrast to theoretical prediction based a preweld heat treatment that reduced grain boundary liquid film thickness did not result in a lower HAZ cracking, which can be related to concomitant reduction in the ability of the base alloy to relax welding stress. In addition, formation of intergranular M₅B₃ boride particles in preweld alloy appeared to have aided cracking susceptibility by lowering grain boundary liquation temperature and widening the brittle temperature range in the HAZ during cooling. Based on the analysis of the results, application of a new preweld heat treatment that prevents the formation of the intergranular borides and induces moderate base alloy hardness resulted in a nearly crack free HAZ in laser welded DS Rene 80 superalloy.     

An investigation on thermal, metallurgical and mechanical states in weld cracking of Inconel 738LC superalloy

The influence of various conditions of Inconel 738 superalloy welding or deposition welding has been studied in order to shed light on the coupling between thermal, metallurgical and mechanical states in the heat affected zone (HAZ) in which cracking may occur particularly during welding and post-weld heat treatment. Predominant crack controlling factors have been highlighted thanks to different pre-weld and post-weld heat treatments, in addition to various welding rates and pre-heating prior to welding. These factors are mainly the material ductility related to the morphology and volume fraction of intermetallic precipitates and thermally induced residual stress. It has appeared that reducing thermally induced residual stress could be more effective for preventing cracking than controlling the material ductility in the related zones thanks to adjustment of pre-weld and post-weld heat treatments. With the objective of cracking remediation, welding on preheated parts leads to lower weld power, to reduce significantly thermal gradients, decreases thermally induced stress and impedes cracks formation despite some localized and temporary decrease in alloy ductility.     

热处理对碳纤维/聚酰胺6复合材料界面结晶及力学性能的影响

探究了热处理对聚酰胺6（PA6）在碳纤维（CF）表面的结晶行为及其界面力学性能的影响。利用差示扫描量热法（DSC）、偏光显微镜（POM）观察法等分析手段考察了热处理对PA6在CF表面结晶行为的影响,揭示了在热处理过程中,PA6进行链段重排,形成小且不完善的新结晶,导致结晶度的上升以及界面横晶形貌的完善;进一步通过单丝微球脱粘实验和单向CF/PA6复合材料横向拉伸实验考察了热处理对PA6与CF的界面结合性能的影响,揭示了经退火热处理的试样由于弱界面和应力集中的减少使界面剪切强度增加且单位体积断裂能下降。     

Effect of heat treatment on creep and fracture behaviour of 1·25Cr–O·5Mo steel

Abstract

The effect of tempering treatments on the microstructure and creep behaviour of multipass 1·25Cr–0·5Mo steel weldments has been evaluated. While tempering invariably reduced the hardness, significant changes in microstructure were only found after heat treatment at 750°C. In this case ferrite bands developed adjacent to the fusion boundary. Tempering increased creep deformation and reduced failure lifetime for base metal specimens. For crossweld testpieces, the susceptibility to low ductility failures in the heat affected zone was found to be linked to the development of creep cavities and cracks. Thus, brittle failure modes were a function of stress, temperature, microstructure, post weld heat treatment, and to a lesser extent, specimen geometry.

MST/3069     

Residual stress driven cracking in superalloy weldments

Superalloy weldments are normally given post weld heat treatments to homogenize the weld metal microstructure, relieve residual stress, and precipitate strengthening phases. The relationship between microstructure and post weld heat treatment is easily studied; it is less straightforward to study the effects of post weld heat treatment on residual stress relaxation. Using a self-restrained testing procedure, a relatively simple approach was used to investigate the effects of microstructure and post-weld heat treatment on cracking during residual stress relaxation. Candidate superalloys for Advanced Ultra Supercritical steam plants were studied. It was found that cracking due to residual stress relaxation is primarily dependent on grain size, and in cases of intermediate grain size, intragranular precipitation is a controlling factor. These results are in agreement with traditional stress relaxation cracking theories.     

Effect of boron and phosphorus on HAZ microfissuring of Allvac 718 Plus superalloy

Abstract

Heat affected zone (HAZ) microfissuring was investigated in electron beam welds of two Allvac 718 PLUS (718 Plus) alloys with different boron and phosphorus contents. Two preweld solution heat treatments were used and microfissuring susceptibility was evaluated by measuring the HAZ crack lengths in both alloys after bead on plate welding. The segregation behaviour of boron and phosphorus was studied before and after welding by secondary ion mass spectrometry (SIMS). Hot ductility behaviour of the alloys in the standard solution preweld heat treatment was also examined. The results of crack measurements, segregation studies and hot ductility tests correlated well with each other. It was observed that besides constitutional liquation of Nb rich MC type carbides, segregation of B and P largely influenced the microfissuring susceptibility of the alloy. Heat affected zone microfissuring increased with an increase in B + P concentrations and with an increase in the preweld solution heat treatment temperature. Segregation behaviour of B and P and its contribution to microfissuring in Allvac 718 Plus alloy were discussed.     

The effect of rejuvenation heat treatments on the repair weldability of wrought Alloy 718

Extensive precipitation of needle- and plate-shaped δ-phase in the γ-nickel matrix of wrought Alloy 718 is the major microstructural change resulting from multiple weld repair/post weld heat treatment (PWHT) cycles. Isothermal heat treatments at 954 °C for times up to 100 h were used to simulate the multiple PWHTs in laboratory samples. Grain size did not change appreciably during these heat treatments owing to δ-phase pinning of the grain boundaries (GBs). The susceptibility of Alloy 718 to heat-affected-zone (HAZ) liquation cracking degraded as a result of these heat treatments. This degradation is due to the short time, high temperature GB liquation caused by the combined effects of δ-phase dissolution, boron carbide constitutional liquation, and GB segregation. A rejuvenation treatment (1010 °C/2 h) effectively restored the degraded weldability by removing the adverse influence of δ-phase through dissolution above the δ-phase solvus. This heat treatment also promoted a spontaneous grain refinement and increase in the fraction of special GBs owing to the elimination of δ-phase pinning of GBs. The combined effects of δ-phase, grain size and fraction of special GBs are discussed in the context of HAZ liquation cracking that occurs during repair welding of Alloy 718.     

Adaptation of aluminium foam properties by means of precipitation hardening

Abstract

Aluminium foam samples based on four aluminium alloys were investigated with respect to their reaction to heat treatments, namely precipitation hardening treatments. Foam samples were produced according to the powder compact foaming or Fraunhofer process. 6000 and 7000 series alloys containing significant amounts of copper (6061, 7075) were compared to members of the same group with lower copper content (6082, 7020) as matrix alloys. Comparison was based on strength values and failure modes as reflected in the stress–strain curves obtained in quasi-static compression tests. Measurements were performed on samples without heat treatment and samples subjected to different precipitation hardening treatments. To evaluate the influence of quench sensitivity, the quench rate was varied for the alloys 6082 and 7020 by using air and water as quenchants.     

THE EFFECT OF INTERMEDIATE HEAT TREATMENTS ON OVERLOAD INDUCED RETARDATIONS DURING FATIGUE CRACK GROWTH IN AN Al-ALLOY

Abstract— Crack growth fatigue tests were carried out on 2024-T3 specimens. Constant-amplitude loading was periodically interrupted by 10 overload cycles. Intermediate heat treatments (T4) were applied to remove the residual stress in the crack tip zone and the crack closure wake behind the crack tip. Retardation effects induced by crack closure due to the previous load history were fully erased by the heat treatments. Overload effects were easily introduced again by new overload cycles afterwards. Crack growth rate results and fractographic observations indicate that primary crack tip plastic deformation (in virgin material) is more effective for crack extension than secondary plastic deformation in an existing plastic zone. This conclusion is significant for cycle-by-cycle crack growth prediction models for variable-amplitude loading.     

An investigation of Stress corrosion cracking in MgAZ61 alloy in 3.5% NaCl + 2% K2CrO4 aqueous solution at room temperature

Effect of specimen orientation, heat treatment and applied potential on the stress corrosion susceptibility of magnesium AZ61 (Mg-6.3% Al-0.5% Zn-0.20% Mn) alloy in an aqueous 3.5% NaCl + 2% K₂CrO₄ solution at room temperature was investigated. Stress corrosion times to failure were measured at different values of initial stress intensities using single edge (pre) cracked sheet tensile specimens and a modified tensometer. It was observed that while the specimen orientation has a significant effect on the measured values of stress corrosion threshold stress intensity, K_Iscc, the effect of varying the quench rate during heat treatment was minimal. Polarization measurements both in stressed and unstressed conditions, failed to reveal any significant effect of the applied stress intensity on the anodic polarization behavior of the alloy. However, measurements made under four different potentiostatic conditions showed a considerable increase in stress corrosion times to failure of the alloy. The results, together with fractographic observations of fractured specimen are discussed in terms of the mechanisms of stress corrosion cracking, in magnesium alloys.     

Mechanical properties evolution during post-welding-heat treatments of double-lap Friction Stir Welded joints

The present study focuses on double-lap Friction Stir Welded (FSW) joints in 2024T3 and 7075T6 aluminium alloys subjected to several post-welding-heat treatments at warm (typical aging) and high temperature (solution range) followed by room temperature deformation (tensile tests). The effect of post-welding-heat treatments on the microstructure and mechanical properties of double lap FSW joints were investigated. Polarized Optical Microscopy (POM) and Scanning Electron Microscopy (SEM) analysis reveal a progressive change in grain size and morphology in high temperature post-welding-heat treated joints, leading to Abnormal Grain Growth in the stir zone. Stress–strain curves are rather flat for 200° and 300 °C post-welded heat treated joints while, for the other set of samples, stress increases with strain to reach maximum stress of 140–160 MPa. Micro-hardness profiles measured on transversal sections of post-welded heat treated joints reveal conditions (temperature and time) of hardness homogeneity at top, bottom and central nugget zone and/or along the whole measured profile. When homogeneity is reached, fracture occurs in the nugget. A relationship between hardness and tensile properties has been applied in the nugget.     

Residual stresses in two laser surface melted stainless steels

Abstract

Residual stresses were studied in two laser surface melted stainless steels: one martensitic, Fe–12Cr–0·2C, and the other austenitic, Fe–17Cr–11Ni–2·5Mo (compositions in wt-%). Stresses were measured by X-ray diffractometry over a range of depths, processing conditions, and stress relieving heat treatments. The volume increase associated with the martensitic transformation develops compressive stresses in single tracks of the martensitic steel and modifies the subsurface stresses of the laser surface melted steel. However, interactions between tracks offset the compressive surface stresses at all but the slightest overlaps. Residual stresses in the martensitic steel are minimized by increasing the advance between tracks and are reduced to a lesser extent by increasing the beam diameter and decreasing the traverse velocity. The austenitic steel, undergoing no solid state phase transformation on cooling, develops tensile residual stresses of the order of the yield stress for all the processing conditions evaluated. Suitable stress relieving heat treatments were identified for each steel.

MST/422     

Acid attack on pore-reduced cements

Because the durability of high-performance cements is as important as their strength, the performance of pore-reduced cement (PRC) in aggressive media such as sulfuric, hydrochloric and ethanoic acids, was studied and compared with that of ordinary Portland cement (OPC). The effects of exposure to these media on these cements were monitored by periodic visual inspection and sample weighing. Specific interactions with regard to interconnected porosity were addressed and the corrosion products characterized. PRC is less susceptible than OPC against hydrochloric and ethanoic acids. However, sulfuric acid damages PRC and OPC to almost the same extent. It is shown by electron microprobe analysis that the hydrochloric and ethanoic acids quickly penetrate the interior of normal cement pastes by acid leaching of the interconnected porosity. The reduced porosity of PRC reduces the susceptibility to attack by this mechanism. Sulfuric acid exposure causes extensive formation of gypsum in the cement surface regions, which results in mechanical stress and ultimately leads to spalling. Thus fresh surfaces are exposed regularly and therefore the relatively closed microstructure of PRC is no hindrance to this kind of attack. This revised version was published online in November 2006 with corrections to the Cover Date.     

50th Anniversary Article: The Origin and Management of Residual Stress in Heat‐treatable Aluminium Alloys

The presence of macroscopic residual stresses in heat‐treatable aluminium alloys can give rise to machining distortion, dimensional instability and increased susceptibility to in‐service fatigue and stress corrosion cracking. This paper presents and reviews details about the residual stress magnitudes and distributions introduced into wrought aluminium alloys by the thermal operations associated with heat treatment. Experimental measurement data and the results of finite element analysis are presented and discussed. The available technologies by which residual stresses in aluminium alloys can be relieved are reviewed. The limitations of these techniques are described, and recommendations are made as to selecting the most appropriate technique to manage residual stresses. Opportunities for the future optimisation of these techniques are also presented.     

Critical Assessment 21: oxygen-assisted fatigue crack propagation in turbine disc superalloys

Ni-based superalloys in turbine disc applications face increasing susceptibility to oxygen-assisted fatigue crack propagation due to increased turbine entry temperatures. The continued lack of understanding of the interplay between the factors operating during oxygen-assisted fatigue crack propagation limits: (1) development of lifing methodologies to accurately predict the fatigue performance of disc alloys/components and (2) associated disc alloy developments. An underpinning requirement to better understand the role of oxygen is to characterise the process of oxygen diffusion in the localised stress/strain state at the crack tip, which is related closely to microstructural features. The link between three-dimensional crack tomography, crack propagation rate and oxygen-related attack needs to be established. Quantitative models which include the interaction between fatigue–creep–oxygen attack need further development.     

High-strength magnesium alloys for degradable implant applications

This article describes the design principles deployed in developing high-strength and ductile Mg-Zn-Zr-Ca-Mn(-Yb) alloys based on a concept, which aims to restrict grain growth considerably during alloy casting and forming. The efficiency of the development approach is discussed. Moreover, the microstructure and phase analysis of the alloys subjected to different thermal treatments are presented and the influence of the alloy composition, particularly the addition of Yb, on the evolution of the microstructure is discussed in connection with the mechanical properties of the materials. The newly developed alloys exhibit high strength (yield stress of up to 350 MPa) at considerable ductility (elongation to fracture of up to 19%) in the as-extruded state and reveal age hardening potential (increase in hardness of 10-15% compared to that in the recrystallization heat-treated state). Appropriate heat treatments enable tailoring of the strength-ductility relation. Thermal annealing of the material resulted in a remarkable increase in ductility (elongation to fracture of more than 20% for all heat-treated samples) while high strength is retained (yield stress ranging from 210 to 315 MPa). We attribute the attractive mechanical properties of the developed alloys to their fine-grained microstructure, where the grain boundaries and lattice defects are stabilized by second phase particles formed during casting and thermal treatments.     

Management of climatic heat stress risk in construction: A review of practices,methodologies, and future research

Climatic heat stress leads to accidents on construction sites brought about by a range of human factors emanating from heat induced illness, and fatigue leading to impaired capability, physical and mental. It is an occupational characteristic of construction work in many climates and the authors take the approach of re-engineering the whole safety management system rather than focusing on incremental improvement, which is current management practice in the construction industry. From a scientific viewpoint, climatic heat stress is determined by six key factors: (1) air temperature, (2) humidity, (3) radiant heat, and (4) wind speed indicating the environment, (5) metabolic heat generated by physical activities, and (6) “clothing effect” that moderates the heat exchange between the body and the environment. By making use of existing heat stress indices and heat stress management processes, heat stress risk on construction sites can be managed in three ways: (1) control of environmental heat stress exposure through use of an action-triggering threshold system, (2) control of continuous work time (CWT, referred by maximum allowable exposure duration) with mandatory work-rest regimens, and (3) enabling self-paced working through empowerment of employees. Existing heat stress practices and methodologies are critically reviewed and the authors propose a three-level methodology for an action-triggering, localized, simplified threshold system to facilitate effective decisions by frontline supervisors. The authors point out the need for “regional based” heat stress management practices that reflect unique climatic conditions, working practices and acclimatization propensity by local workers indifferent geographic regions. The authors set out the case for regional, rather than international, standards that account for this uniqueness and which are derived from site-based rather than laboratory-based research.     

Residual stresses and structural changes generated at different steps of the manufacturing of gears: Effect of banded structures

Banded ferrite-pearlite structures, and in general chemically inhomogeneous structures, react non uniformly to elevated temperatures during forging and/or subsequent heat treatment processes, affecting the final stress state (plastic deformation is required to accommodate dissimilar thermal expansion behavior for each phase) and consequently leading to distortions. These unpredicted distortions are one of the major causes of rejected components and components that need to be reworked, leading to production losses.The aim of the present research work is to study the effect of forging and different thermal treatments (normalizing, quenching and tempering), i.e., the effect of different steps of the manufacturing of gears, on the final residual stress state, microstructure and hardness of AISI 4140 steel, a material that frequently presents ferrite-pearlite banded structures coming from segregation of alloying elements (such as chromium and carbon). With this purpose, portions of a forged AISI 4140 steel ring have been subjected to different thermal treatments. Residual stresses, hardness and microstructure after each treatment (forging, normalizing, quenching and tempering) have been studied experimentally and compared with the predictions of FEM simulations of heat treatment processes.     

高温处理对3D C/SiC复合材料热膨胀性能的影响

研究了不同高温处理前后3D C/SiC复合材料热膨胀系数(CTE)的变化规律,从材料内部热应力变化及结构改变的角度定性地分析了其变化机理。研究发现,3D C/SiC复合材料的热膨胀系数受界面热应力的影响,其变化规律是纤维和基体相互限制、相互竞争的结果;高温处理可提高材料的热稳定性,并通过改变界面热应力及材料内部结构,来影响材料热膨胀系数的变化规律;通过增加基体裂纹来降低复合材料的低温热膨胀,但不影响其变化规律;通过改变材料内部结构,使热应力发生变化并重新分布,对复合材料的高温热膨胀产生显著影响。但高温处理没有改变3D C/SiC复合材料的基体裂纹愈合温度(900℃)。