Effect of Post-weld Heat Treatment on Microstructure and Fracture Toughness of 30CrMnSiNi2A Steel Welded Joints

The electron beam local post-weld heat treatment (EBLPWHT) is a rather new method that provides the advantages of high precision, flexibility and efficiency, energy saving and higher productivity. This paper studies the effect of two post-weld heat treatment processes on the microstructure, mechanical properties and fracture toughness of an electron beam welded joints in 30CrMnSiNi2A steel. EBLPWHT, in a vacuum chamber, immediately after welding and a traditional furnace whole post-weld heat treatment (FWPWHT) were compared. The experimental results show that, after EBLPWHT treatment, the main microstructure of weld was changed from coarse acicular martensite into lath rnartensite, HAZ was changed from lath martensite, bainite into lower bainite, and base metal was changed from ferrite and pearlite into upper bainite and residual austenite. The microstructures of different zones of joints in FWPWHT condition were tempered sorbite. The properties of welded joints can be improved by the EBLPWHT in some extent, and especially largely for the fracture toughness of welded joints. However the value of fracture toughness of base metal is comparatively low, so appropriate heat treatment parameters should be explored in the future.     

热输入对10CrNi3MoV钢MAG焊接头组织和性能的影响

为制定合理的焊接工艺,保证焊接质量,设置不同焊接热输入进行了10CrNi3MoV钢MAG焊接。采用微观组织分析、断口观察、力学测试等手段研究了焊接热输入对接头组织及性能的影响。结果表明,热输入较小时（E=11.0 kJ·cm^-1、E=14.4 kJ·cm^-1）,焊缝组织以针状铁素体为主,并含有部分粒状贝氏体、先共析铁素体等;热输入较大时（E=18.1 kJ·cm^-1）,针状铁素体占比降低,粒状贝氏体、先共析铁素体等增多,组织粗化。随热输入的增大,粗晶区晶粒粗化,组织由板条马氏体逐步转变为板条贝氏体,板条界限模糊,并有粒状贝氏体出现;焊缝金属强度降低,冲击韧性先略有升高后显著降低,断裂形式由微孔聚缩型韧断变为准解理/韧性混合断裂。热输入E=14.4 kJ·cm^-1时,焊缝组织以细密的针状铁素体为主,具有最佳强韧性匹配。     

Predicting long-term creep failure of bimodal polyethylene pipe from short-term fatigue tests

Short-term fatigue testing was used to predict long-term creep failure of a bimodal polyethylene (BMPE) pipe with superior creep resistance. The stepwise crack propagation was studied by increasing the R-ratio (defined as the ratio of the minimum to the maximum stress intensity factor in the fatigue loading cycle) at 50 °C from 0.1 approaching creep (R = 1). Crack growth rate (da/dt) was related to the maximum stress intensity factor K _I,max and R-ratio by a power law relationship \frac\textda\textdt = B^￠ K_\textI,max⁴ (1 + R) ^{- 8.5} {\frac{{{\text{d}}a}}{{{\text{d}}t}}} = B^{\prime } K_{{{\text{I}},\max }}^{4} (1 + R)^{ - 8.5} . The correlation in crack growth kinetics allowed for extrapolation to creep fracture from short-term fatigue testing. The temperature dependence of crack growth rate was contained in the prefactor B′. A change in slope of the Arrhenius plot of B′ at 67 °C indicated that at least two mechanisms contributed to crack propagation, each dominating in a different temperature region. This implied that a simple extrapolation to ambient temperature creep fracture from elevated temperature tests might not be reliable.     

Creep Behavior of Dispersion-Hardened Aluminum Materials

Dispersion-hardened aluminum materials of pure aluminum with extremely fine oxide and carbide dispersions and very fine grain sizes were creep-deformed under compressive loadings between 573 and 773 K. The creep behavior of the investigated materials is influenced by time, temperature, stress level and microstructure. An increasing content of dispersions causes increasing threshold stresses _thand resistances against creep. The Norton plots of the minimum creep rate versus stress are characterized by extremely high stress exponents n. On the basis of the threshold concept it is demonstrated that the same diffusion process dominates in the dispersion-hardened aluminum materials as in pure aluminum. Their true stress exponents n^*as the slopes of the best fit lines of the are close to 5. The threshold stress decreases considerably with increasing temperature due to the thermally activated recovery of long-range internal back stresses of quasi-planar dislocation structures on the grain boundaries.     

具有合理硬度梯度和组织分布的渗碳钢23CrNi3Mo的热处理冷却行为

利用Gleeble-1500热模拟机、光学显微镜(OM)、扫描电镜(SEM)以及透射电镜(TEM)对渗碳钢23CrNi3Mo的连续冷却相变规律以及等温转变规律进行了研究,并基于此,设计了一种新的热处理冷却工艺。研究结果表明,渗碳后试样以0.05℃/s和0.1℃/s的冷速连续冷却时,表面渗碳层为高碳马氏体组织,过渡区为高碳马氏体+下贝氏体的混合组织,基体为下贝氏体组织;渗碳试样外表面在高温段以较低的冷速(0.05~3℃/s)连续冷却时,碳化物沿晶界析出形成网状碳化物;无渗碳的实验钢的贝氏体等温转变温度范围为375~450℃。新的热处理冷却工艺为:试样在880℃保温完成后,采用快速冷却工艺,以冷速大于等于5℃/s进入贝氏体转变温度区,直接入450℃的盐浴炉,入炉后均温5~10min,在低温转变区即贝氏体转变温度区间,采用慢速冷却工艺,冷速小于等于0.1℃/s。获得的试样渗碳层深度为1.4mm,国外的阿特拉斯钎头渗碳层深度为1.2mm,两者基本相同,但前者硬度分布更加平缓;两者表面显微组织均为高碳马氏体组织,过渡区均为马氏体加下贝氏体组织,基体均为贝氏体组织。通过设计新的热处理冷却工艺,获得了与国外钎头相同水平的试样。     

Microstructure and creep behavior of a new developed nickel‐base single‐crystal superalloy

A new developed nickel‐base superalloy is employed to prepare single‐crystal castings. The as‐cast and heat‐treated microstructure, the creep behavior in 900 °C/ 500 MPa and 1100 °C / 140 MPa are investigated. The dendrites are well developed in the as‐cast microstructure. Segregation and γ′ precipitation morphology difference exist between dendrite core and γ/γ′ eutectic pool. After heat treatment the segregation is significantly decreased and the γ′ precipitations of all regions become much more uniform and cubic. Creep curves in varied conditions both present three stages, while the specimen in 1100 °C exhibits lower creep rate and longer life span. The fracture analysis shows that fracture style changes from mixed fracture to ductile when the temperature increases from 900 °C to 1100 °C. Porosity is the crack resource to form the facet, which is observed in both specimens. The γ′ precipitations raft in both specimens, while the further development of γ′ rafting is found in that of 1100 °C / 140 MPa, as well as the surface recrystallization with coarser rafted γ/γ′ microstructure.     

一种4.5%Re镍基单晶合金在980℃蠕变期间的变形与损伤机制

通过蠕变性能测试和组织形貌观察,研究了一种Re含量为4.5%Re(质量分数,下同)的镍基单晶合金的高温蠕变行为、变形和损伤机制。结果表明,4.5%Re合金在980℃/300MPa的蠕变寿命为169h。蠕变初期,合金中立方γ′相转变为垂直于应力轴的N型筏状结构。稳态蠕变期间,合金的变形机制为位错在基体中滑移和攀移越过筏状γ′相。蠕变后期,合金的变形机制为位错在基体中滑移和剪切进入筏状γ′相。由于γ基体通道较窄,位错在基体通道中滑移所需的阻力较大。剪切进入γ′相的110超位错可由{111}面交滑移至{100}面,形成K-W锁,从而抑制位错的滑移和交滑移,这是合金具有较好蠕变抗力的主要原因。主/次滑移位错的交替开动,可致使筏状γ′相扭曲,并促使裂纹在筏状γ/γ′两相界面萌生;裂纹沿垂直于应力轴方向扩展,直至断裂,这是合金的蠕变断裂机制。     

Very strong low temperature bainite

Abstract

Bainite has been obtained by heat treatment at temperatures as low as 125°C in a high carbon, high silicon steel. This has had the effect of greatly refining the microstructure, which is found to have a strength in excess of 2.5 GPa together with an ability to flow plastically before fracture. Such properties have never before been achieved with bainite. In this paper metallographic details are reported of the very fine bainitic microstructure associated with the incredibly low transformation temperature, where during the time scale of the experiments, an iron atom cannot diffuse over a distance greater than ~ 10^-17 m. Yet, the microstructure has a scale in the micrometre range, consistent only with a displacive mechanism of transformation.     

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。     

高强度结构钢及其接头热影响区的疲劳及断裂性能分析

采用焊条电弧焊对48 mm厚高强度结构钢进行焊接,对焊接接头热影响区疲劳裂纹扩展门槛值ΔK_th、裂纹扩展速率da/dN和断裂韧度K_IC进行研究并与基体进行对比。结果表明,在室温下,焊接接头热影响区具有更好的疲劳和断裂性能;随着与熔合线距离的增大,热影响区的组织依次为粗大板条状贝氏体+奥氏体薄膜、细粒状贝氏体、回火索氏体+细粒状贝氏体,硬度逐渐下降;在室温下,焊接接头热影响区和基体冲击韧性均位于上平台。热影响区的残余奥氏体薄膜和硬度较高的贝氏体是影响其疲劳和断裂性能的重要因素。     

Creep behavior of MAR-M 302 Co-base superalloy

MAR-M 302 a Co-base superalloy is currently used for nozzle guide vanes and stator blades in military and commercial aircraft-turbine engines. These components are usually operated at 1073–1273 K. In this study, the uniaxial constant load creep behavior of MAR-M 302 superalloy over temperature range of 1088–1198 K and stress range of 155–321 MPa was investigated. Concerning creep as thermally activated phenomenon, the investigated alloy followed empirical equation [(e)\dot] = Asⁿ exp( - \fracQRT ) \dot{\varepsilon } = A\sigma^{n} \exp \left( { - \frac{Q}{RT}} \right) with n values ranging mainly from 7 to 10. The mean apparent activation energy was calculated as 525 ± 29 kJ/mol. The correlation between rupture life and creep characteristics was studied by different prediction methods that are based on the iso-stress lines and those relating the secondary or steady state creep rate to rupture life.     

Longitudinal Creep Behaviour of a SiC/Ti-6242 Composite in a Vacuum Atmosphere

This study deals with the longitudinal creep behaviour of a unidirectionally reinforced SM1140+/Ti-6242 composite at 500C in a vacuum atmosphere. The monitoring of acoustic emission and microstructural observations show that filament fracture develops during the creep test. This damage results from the stress relaxation in the matrix, which increases the axial stress in the filament. Time till the creep rupture can be predicted with a model describing the filament damage accumulation.     

Influence of heat treatment on creep of a Mn–N stabilised austenitic stainless steel

Creep at 700 °C/196 MPa and 900 or 925 °C/27.4 MPa of 21Cr–4Ni–9Mn austenitic stainless steel is determined as a function of the heat treatment. The heat treatment variation involves altering the solution heat treatment cooling rate from water quenching to cooling at 6 or 4 °C/min causing: serrated grain boundaries versus planar grain boundaries, coarser intergranular carbides, and discontinuous precipitation of grain boundary reaction zones. Water quenching causes improved creep resistance. Creep fracture and cracking is intergranular. Coarse intergranular carbides and grain boundary reaction zones cause premature void formation and cracking, this damage leading to an accelerating creep rate and lowering creep resistance of the more slowly cooled conditions. During creep, grain boundary serrations, which may otherwise contribute to improved creep, are eliminated. Determining the individual influence of grain boundary serrations on creep requires a detailed investigation of various heat treatment parameters to prevent concurrent formation of grain boundary reaction zones and serrations.     

LIFE PREDICTION OF 316 STAINLESS STEEL UNDER CREEP-FATIGUE LOADING

Life prediction for creep-fatigue loading conditions should be related to creep damage mechanisms. In order to examine the effect of the creep damage mode on rupture life under creep-fatigue loading, a “combined creep-fatigue loading test” was carried out on 316 stainless steel. In this method, creep loading and fatigue loading are repeated alternately. The fracture criteria under combined loading closely depend on the creep fracture modes of the static creep test. A new life prediction method which uses this new fracture criterion is proposed. The criteria are changed when the creep damage mode varies. In order to verify the adequacy of this method, fatigue tests with a tensile strain-hold wave form were carried out. It is clear that rupture life in such fatigue tests is dependent on the chosen fracture criteria.     

一种GH4169镍基合金的组织结构与蠕变性能

通过对一种等温锻造GH4169镍基合金进行直接时效处理,蠕变性能测试及组织形貌观察,研究了该合金的组织结构与蠕变行为。结果表明,GH4169合金的组织结构由γ基体,γ′相、γ″相和δ相组成,且各相之间保持共格界面。测定出合金在660℃/700MPa条件下的蠕变寿命为123h。合金在680℃/700MPa的蠕变寿命为39h,在实验温度和应力范围内,计算出直接时效合金的蠕变激活能为588.0kJ/mol。合金在蠕变期间的变形机制是位错滑移和孪晶变形,其中,沿晶界析出的粒状碳化物,可抑制晶界滑移,是使合金具有较好蠕变抗力的主要原因。随蠕变进行,开动的滑移系中位错运动至晶界受阻,并塞积于该区域引起应力集中,当应力集中值大于晶界的结合强度时,可促使其在与应力轴垂直的晶界处发生裂纹的萌生与扩展,直至断裂,是合金在蠕变期间的断裂机制。     

Creep ductility considerations for high energy components manufactured from creep strength enhanced steels

Abstract

It is well established that the tendency for low ductility ‘creep brittle’ fracture behaviour in tempered martensitic steels is linked to the formation and growth of micro voids or ‘cavities’. Details of the contributions of all factors affecting damage development are still under investigation. However, it is known that for tempered martensitic steels voids often initiate over most of the creep life. Nucleation has been recorded on both prior austenite grain boundaries and at other micro structural features such as lath boundaries. The number of voids formed, and the fracture behaviour observed, depend on the type of creep strength enhanced ferritic (CSEF) steel and specific details of fabrication and heat treatment. In Grade 91 steel, void nucleation is sensitive to metallurgical factors such as composition and steel making practices. Key indicators of susceptibility to creep cavitation also include the levels of trace elements present and the presence of hard non-metallic inclusions. In Grade 92 steel, creep void formation has been linked to boron nitrides and other inclusions. These inclusions are present when there has been insufficient control of composition and heat treatment. Metallurgical factors linked to whether a particle will nucleate a void include the nature of the inclusion/matrix interface, the shape and size and the location of the inclusions within the microstructure. This paper describes the results of critical uniaxial and multiaxial testing for CSEF steels and compares data from nominally the same steels which have different metallurgical susceptibilities to void formation.     

The effects of hydrostatic pressure on the ductility of metals and alloys

A law governing the change in the ductility of metals and alloys under pressure is given: $$\begin{gathered} \left( {\frac{P}{{\sigma _n }}} \right) = \tfrac{1}{2}\frac{1}{{\sigma _n }}\frac{{d\sigma }}{{d\varepsilon }}\{ \varepsilon _{local} (P)^{\tfrac{3}{2}} - \varepsilon _{local} (O)^{\tfrac{3}{2}} \} + \tfrac{1}{3}\frac{1}{{\sigma _n }}\frac{{d\sigma }}{{d\varepsilon }}\{ \varepsilon _{local} (P) - \varepsilon _{local} (O)\} + \hfill \\ {\text{ }} + \tfrac{1}{2}\{ \varepsilon _{local} (P)^{\tfrac{1}{2}} - \varepsilon _{local} (O)^{\tfrac{1}{2}} \} \hfill \\ \end{gathered}$$ where P is the hydrostatic pressure, ?_local is the strain accumulated from the start of necking to fracture, σ _n necking stress and (dσ/d?) the coefficient of linear work hardening. This relation is derived from a newly proposed criterion of ductile fracture, viz. “constancy of hydrostatic tensile stress”, which indicates that the change of ductility with pressure obeys a three halves power law. The observed increase in ductility of widely differing metals and alloys under pressure up to 10,000 kg/cm⁴ has confirmed that the proposed criterion is acceptable. It is further shown that the ductilities of some copper alloys with low stacking fault energy, such as Cu-Zn and Cu-Ge alloys, increases with pressure at the beginning but the increase stops at fairly low pressure, i.e. 3,500 ～ 4,000 kg/cm², and their ductilities become almost insensitive to the pressure applied. It is suggested that ductile fracture of metals with low stacking fault energy is dominated by a process which occurs not by the hydrostatic stress component but by shear stress only.     

The constant stress tensile creep behaviour of a superplastic zirconia-alumina composite

A detailed study was conducted to evaluate the constant stress tensile creep behaviour of a superplastic 3 mol% yttria-stabilized zirconia-20 wt% alumina composite. The comprehensive experimental results indicate that creep deformation may be expressed in the form exp(–585 000/8.3T), where is the steady-state creep rate, is the imposed stress, is the linear intercept grain size andT is the absolute temperature. Microstructural observations revealed that there is very little dislocation activity, or change in grain size or shape. A detailed analysis was conducted to evaluate the possible rate-controlling mechanisms in terms of the experimentally determined mechanical properties and the microstructural observations. Based on the maintenance of an equiaxed microstructure and the strong grain size and stress dependence, it is concluded that creep occurs by a grain-boundary sliding/grain rearrangement process.     

Effects of heat treatment on mechanical properties of high-chromium nickel-base superalloy IN 939

Abstract

The effects of initial heat treatment on the tensile and creep properties of alloy IN 939 are described and explained in terms of microstructural and microcompositional variations. It is demonstrated that a substantial increase in room temperature properties and in short-term high-temperature properties can be obtained by a modified three stage, 14 h heat treatment. Long-term elevated-temperature creep properties are found to be insensitive to initial heat treatment, since in this case the evolution of the microstructure is largely determined by test conditions. A comparison is made between IN 939 and IN 738, and it is shown that the very long-term high-temperature creep properties of the two alloys are closely similar. The selection of heat treatments to obtain optimum properties over the entire range of temperature and stress conditions encountered within a turbine blade is also discussed.

MST/434