Tem observation and fracture morphology in the cghaz of a new 0cr18mo2ti ferritic stainless steel

Microstructure, precipitates and fracture morphology in the coarse grained heat-affected zone CGHAZ) of a new high-purity 0Cr18Mo2Ti ferritic stainless steel were studied by means of optical metallography, SEM, TEM, X-ray diffractometer, etc. Experimental results indicated that grain coarsening resulted in brittle fracture in the CGHAZ of 0Cr18Mo2Ti steel. The reduction of impact toughness in the CGHAZ due to change of cooling rate can be attributed to the increase of nitrides (TiN, Cr₂N, etc). These nitrides in the CGHAZ promote initiation and propagation of brittle cracks. The precipitated Cr₂N nitrides in the grain boundaries decrease impact toughness in the CGHAZ of 0Cr₁₈Mo₂Ti steel by promoting crack initiation. In practical applications, the welding heat input (E) should be as low as possible to prevent toughness reduction in the CGHAZ.     

氮对钒微合金钢粗晶热影响区(CGHAZ)的组织和性能的影响

用热模拟方法研究了氮含量对钒微合金钢粗晶热影响区(CGHAZ)的组织和性能的影响。结果表明,氮含量为0.0031%或0.021%时,CGHAZ的韧性较差。氮含量0.0031%时CGHAZ中有少量的Ti(C,N),晶界铁素体(GBF)较少,晶内有大量尺寸较大的侧板条铁素体(FSP),解理裂纹沿FSP的直线扩展使其韧性较差。氮含量0.021%时在CGHAZ中生成了较为粗大的(Ti, V)(C, N)和GBF,解理裂纹沿GBF扩展使其韧性较差。氮含量为0.012%时低温韧性较好,在CGHAZ中生成了大量细小的(Ti, V)(C, N)粒子,且GBF尺寸相对较小,晶内有大量的针状铁素体(AF)。这些因素都有利于阻止裂纹扩展,使其低温韧性显著提高。     

Microstructure and toughness of coarse grain heat-affected zone of domestic X70 pipeline steel during in-service welding

The microstructures and mechanical properties of coarse grain heat-affected zone (CGHAZ) of domestic X70 pipeline were investigated. The weld CGHAZ thermal cycles having different cooling time Δt _8/5 were simulated with the Gleeble-1500 thermal/mechanical simulator. The Charpy impact absorbed energy for toughness was measured, and the corresponding fractographs, optical micrographs, and electron micrographs were systematically investigated to study the effect of cooling time on microstructure, impact toughness, and fracture morphology in the CGHAZ of domestic X70 pipeline steel during in-service welding. The results of simulated experiment show that the microstructure of CGHAZ of domestic X70 pipeline steel during in-service welding mainly consists of granular bainite and lath bainite. Martensite–austenite (M–A) constituents are observed at the lath boundaries. With increase in cooling time, the M–A constituents change from elongated shape to massive shape. The reduction of toughness may be affected by not only the M–A constituents but also the coarse bainite sheaves. Accelerating cooling with cooling time Δt _8/5 of 8 s can be chosen in the field in-service welding X70 pipeline to control microstructures and improve toughness.     

The role of Cu and Al addition on the microstructure and fracture characteristics in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels with superior toughness

The effects of Cu and Al addition on the microstructure and fracture in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels with superior toughness were studied and compared with the X70 pipeline base steel counterpart. The microstructure in base steel was dominated by a small fraction of acicular ferrite and predominantly bainite. However, acicular ferrite microstructure was obtained in Cu-bearing steel, which nucleated on complex oxide with outer layer of MnS and CuS because of Cu addition. The microstructure in Al-bearing steel consisted of bainite with ultrafine martensite–austenite constituent, which was refined by Al addition. CGHAZ in Cu-bearing and Al-bearing steels had superior impact toughness and ductile fracture, which were attributed to acicular ferrite and ultrafine martensite–austenite constituent, respectively.     

Effect of weld heat input on toughness and structure of HAZ of a new super-high strength steel

Fracture morphology and fine structure in the heat-affected zone (HAZ) of HQ130 super-high strength steel are studied by means of SEM, TEM and electron diffraction technique. Test results indicated that the structure of HAZ of HQ130 steel was mainly lath martensite (ML), in which there were a lot of dislocations in the sub-structure inside ML lath, the dislocation density was about (0.3∼ 0.9) x 10¹²/cm². No obvious twin was observed in the HAZ under the condition of normal weld heat input. By controlling weld heat input (E ≤20 kJ/cm), the impact toughness in the HAZ can be assured.     

Effect of Cu addition on microstructure and impact toughness in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effects of Cu content on microstructure and impact toughness in the simulated coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels were investigated. It has been observed that the microstructure in the simulated CGHAZ of Cu-free steel is dominated by a small proportion of acicular ferrite and predominantly bainite with martensite–austenite constituent. Whereas, in the 0.45 and 1.01% Cu-containing steels, the acicular ferrite increased significantly due to the effective nucleation on intragranular inclusions with outer layer of MnS and CuS. The formation of acicular ferrite is attributed to superior high heat-affected zone impact toughness in the 0.45% Cu-containing steel. Furthermore, the increasing martensite–austenite constituent and ε-Cu precipitates in the simulated CGHAZ of 1.01% Cu-containing steel caused degradation in impact toughness.     

Effects of microstructural change on fracture characteristics in coarse-grained heat-affected zones of QLT-processed 9% Ni steel

This study investigates the correlation between the microstructural change and fracture characteristics in the coarse-grained heat-affected zones (CGHAZs) of the newly developed quenching, lamellarizing and tempering (QLT)-processed 9% Ni steel. The microscopic fracture behaviors of the various sub-zones within the HAZs including local brittle zone (LBZ) were estimated using simulated HAZ specimens. Both results of Charpy impact tests and in situ scanning electron microscopy (SEM) observations on simulated CGHAZ specimens show that the inter-critically reheated coarse-grained HAZ (IC CGHAZ) is a primary LBZ of this steel at cryogenic temperature, but not at room temperature. Microstructural analysis suggests that, unlike in other studies, the cryogenic LBZ phenomenon of the IC CGHAZs cannot be explained simply by the amount of martensite–austenite (M–A) constituents, but is mainly associated with the carbon contents in them. From all results obtained, a mechanism for microscopic toughness change among the CGHAZs is proposed and discussed.     

12CrNi5MoV锻钢焊接热影响区组织与性能研究

采用热模拟技术研究了12CrNi5MoV锻钢焊接热影响区粗晶区的组织与性能。结果表明,经历一次焊接热循环后,材料粗晶区的抗拉强度与原始态试样相比显著升高,而冲击韧性明显降低;冷却过程中冷却速度加快引起材料组织中孪晶马氏体与残余奥氏体含量的增加是其粗晶区力学性能变化的主要原因。     

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.     

析出强化超细晶粒钢焊接热影响区粗晶区的组织和性能

为研究焊接对800 MPa级Ti、Nb复合微合金化析出强化超细晶粒钢组织性能的影响.运用Gleeble3500热模拟试验机,对实验钢进行单道次焊接热循环试验,并研究冷却速度、冷却时间t8/5对焊接热影响区粗晶区(CGHAZ)组织、性能的影响.结果表明:冷却速度5~15℃/s,CGHAZ的组织为贝氏体,冷却速度进一步增大,会出现马氏体.随着冷却时间t8/5的增加,原奥氏体晶粒尺寸逐渐增加,硬度值逐渐降低,冲击韧性先上升后下降.t8/5为20~120 s时,CGHAZ显微硬度(223~250.4 HV)均小于母材的显微硬度(270.6 HV),出现软化现象,t8/5为20 s时,冲击吸收功最高,为18.2 J,但仅有母材的25.3%.经历焊接热循环后,奥氏体晶粒粗化以及CGHAZ出现贝氏体组织是导致脆化的主要原因.     

X90钢级管线钢预精焊接头的性能研究

为综合研究X90管线钢的焊接性,选用国内某钢厂轧制的X90管线钢卷板,利用预精焊工艺制备试验钢管4根,采用金相分析、扫描电镜(SEM)断口分析、夏比V型缺口冲击试验、拉伸、弯曲、硬度等试验,研究了焊接接头各个区域的组织和性能.试验结果表明:内外焊缝区组织均为针状铁素体,热影响区(HAZ)粗晶区晶粒粗化严重,主要组织为粒状贝氏体和贝氏体铁素体,在原奥氏体晶界和贝氏体板条内部存在块状或条状的(马氏体-奥氏体)M-A组元;HAZ冲击功离散性较大,出现了单值较低(45 J)的试样,SEM断口分析呈现典型的解理断裂特征;焊接接头抗拉强度805~815 MPa,断裂位置均在HAZ;焊接接头反弯试样易在HAZ出现裂纹和脆断现象;HAZ硬度在220~250 HV之间,较母材下降30 HV左右.HAZ是X90预精焊钢管焊接接头的薄弱环节,为提高X90管线钢的焊接稳定性,应重点研究精焊内外热循环双热影响亚区的组织转变和脆化机理.     

Statistical scatter in the fracture toughness and Charpy impact energy of pearlitic steel

The effect of microstructure on the fracture characteristics of high carbon hypo eutectoid steel was studied under conditions of quasistatic and dynamic loading. Experimentally determined sets of fracture toughness and Charpy impact energy values were statistically treated. A relationship was found between fracture toughness and Charpy impact energy. In the very brittle domain, the fracture toughness increases slightly with increasing Charpy impact energy. In the domain where the fracture toughness is higher, the rise in fracture toughness with increasing Charpy impact energy is more pronounced. Detailed SEM examination of fractured compact tension (CT) and Charpy V-notch (CVN) specimens showed that the fracture at ambient temperature occurred almost exclusively by cleavage. There were no visible differences in the morphology of cleavage facets on the fracture surfaces of Charpy and CT specimens. Mechanisms of cleavage initiation were revealed by the fractographical investigation of fracture surfaces. Whereas the fracture surfaces of broken CT specimens exhibit a number of cleavage origins, the fracture surfaces of CVN specimens usually show only one.     

Direct metal deposition (DMD) of H13 tool steel on copper alloy substrate: Evaluation of mechanical properties

H13 tool steel powder was clad on copper alloy substrate both directly and using 41C stainless steel (high Ni steel) powder as a buffer layer by direct metal deposition (DMD). Cu-steel bimetallic die casting and injection molding tools are of high interest for reduction of cycle time by efficient heat extraction due to high thermal conductivity of copper. The mechanical properties of these bimetallic structures were investigated in terms of bond strength, impact energy and fracture toughness. The bond interfaces of these claddings showed porous and crack free transition regions. The bond strength was higher in the directly clad H13 tool steel compared to the H13 tool steel clad with 41C stainless steel as buffer layer. The fracture morphology in tensile test specimens showed ductile dimple fracture. Presence of necking just below the interface depicted the softening of substrate in heat affected zone (HAZ) during cladding. The Charpy impact energy is little higher in the 41C stainless steel buffered specimens compared to the directly clad H13 tool steel specimens but the fracture toughness results showed reduction of fracture toughness in the 41C stainless steel buffered specimens due to the low strength in the tensile test. However the fracture toughness value was in the ductile region for both deposits.     

Influence of peak temperature during simulation and real thermal cycles on microstructure and fracture properties of the reheated zones

The objective of this paper is to study the influence of the second peak temperature during real and simulated welding on properties of the subcritically (S), intercritically (IC) and supercritically (SC) reheated coarse grained heat affected (CGHAZ) zones. The X80 high strength pipeline microalloyed steel was subject to processing in a double-pass tandem submerged arc welding process with total heat input of 6.98 kJ/mm and thermal cycles to simulate microstructure of reheated CGHAZ zones. This involved heating to a first peak temperature (T_P1) of 1400 °C, then reheating to different second peak temperatures (T_P2) of 700, 800 and 900 °C with a constant cooling rate of 3.75 °C/s. Toughness of the simulated reheated CGHAZ regions were assessed using Charpy impact testing at 0 °C, −25 °C and −50 °C. The microstructure of the real and simulated reheated CGHAZ regions was investigated using an optical microscope and field emission scanning electron microscope. Morphology of the martensite/austenite (MA) constituent was obtained by the use of a field emission scanning electron microscope. The blocky and connected MA particles, along prior-austenite grain boundaries, act as a brittle phase for the initiation site of the brittle fracture. Charpy impact results indicated that IC CGHAZ had less absorbed energy with higher transition temperature and hardness. The SC CGHAZ region showed higher absorbed impact energy with lower hardness. Design of multipass weld joints with less IC CGHAZ regions can result in a higher toughness property.     

正火型V-N-Ti和Nb-V-Ti海工钢焊接粗晶热影响组织和韧性研究

用焊接热模拟方法研究了V-N-Ti和Nb-V-Ti微合金化正火型海工钢模拟粗晶热影响区(CGHAZ)组织和韧性的变化规律。结果表明,组织的不同使V-N-Ti设计正火型海工钢的模拟CGHAZ韧性比Nb-V-Ti钢的好。对于V-N-Ti钢,较高的N含量提高了富Ti(Ti, V)(C, N)粒子析出温度和铁素体形核能力,使模拟CGHAZ原始奥氏体晶粒和(取向差角为15°)晶粒细化,并生成能阻止或使解理裂纹的偏转细小多边形铁素体,因此具有良好的低温韧性。而Nb-V-Ti钢模拟CGHAZ原奥氏体晶界上的链状M-A、粗大的原始奥氏体晶粒和有效晶粒尺寸,是模拟CGHAZ韧性差的原因。     

热处理工艺对P91耐热钢中δ-铁素体和冲击性能的影响

P91耐热钢热加工(轧制、焊接、热处理)过程中易产生δ-铁素体,且其形态、数量和分布与热加工温度关系密切。通过设计P91耐热钢热处理工艺,在1 150℃、1 250℃、1 300℃温度下正火获得马氏体+δ-铁素体混合组织,并对1 300℃正火组织进行1 050℃(油冷)+760℃(空冷)。采用金相显微镜、显微硬度计和扫描电子显微镜等技术研究δ-铁素体数量、形态、分布的变化,并测试各热处理状态下的冲击韧性和失效模式。结果表明,P91钢随着正火温度升高,δ-铁素体数量增加;形态呈细条状、细条状+块状和多边形块状分布;1 050℃正火+760℃回火不能消除在1 300℃正火时产生的δ-铁素体,但能减少其数量、改变其形态与分布。随δ-铁素体含量增加冲击功减小,冲击断口形貌从韧/脆混合断裂转变为脆性断裂,边界平直的块状多边形δ-铁素体较条状形态更不利于冲击韧性。     

Microstructure and mechanical properties of dissimilar materials joints between T92 martensitic and S304H austenitic steels

In this paper, T92 martensitic steel and S304H austenitic steel were welded by gas tungsten arc welding (GTAW) process. Microstructural features and mechanical properties of T92 and S304H dissimilar materials joints were investigated. The results showed that the part of the joints with relatively weak tensile strength was T92 coarse-grained heat affected zone (CGHAZ), while the part of the joints which revealed relatively weak toughness was weld metal. The decrease of tensile strength in T92 CGHAZ was due to its coarse tempered martensite structure. Weak toughness of the joints was resulted from the coarse dendritic austenite of the weld metal. However, the weld metal in transverse direction of the joints was provided higher tensile strength by the orientation distribution of grains compared with T92 CGHAZ.     

焊接热输入对06CuNiCrMoNb钢HAZ组织与性能的影响

研究了热输入对06CuNiCrMoNb钢焊接热影响区不同部位组织和性能的影响,重点分析了粗晶区的韧性与组织之间的关系。结果表明,模拟焊接热影响区没有出现"软化"现象,但是当线能量大于30kJ/cm情况下,粗晶区低温韧性迅速下降。对粗晶区的分析显示,线能量17kJ/cm条件下贝氏体铁素体呈细小板条状,在板条间存在着残余奥氏体薄膜,随线能量的增大,块状的铁素体数量增多,并且出现不规则片状M+A组元。     

Thermal Activation Analyses of Dynamic Fracture Toughness of High Strength Low Alloy Steels

A formula is derived for determining the influence of temperature and loading rate on dynamic fracture toughness of a high strength low alloy steel (HQ785C) from thermal activation analysis of the experimental results of three-point bend specimens as well as introducing an Arrhenius formula. It is shown that the results obtained by the given formula are in good agreement with the experimental ones in the thermal activation region. The present method is also valuable to describe the relationship between dynamic fracture toughness and temperature and loading rate of other high strength low alloy steels.     

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.