Fracture toughness analysis of heat-affected zones in high-strength low-alloy steel welds

This study is concerned with a correlation of fracture toughness with microstructural factors in heat-affected zones (HAZs) of a normalized high-strength low-alloy (HSLA) steel. In order to explain weld joint performance, tensile and plane strain fracture toughness tests were conducted for the simulated coarse-grained HAZ microstructures. The micromechanisms of fracture processes involved in void and microcrack formation are identified byin situ scanning electron microscopy (SEM) fracture observations and void initiation study. The fracture toughness results are also interpreted using simple fracture initiation models founded on the basic assumption that a crack initiates at a certain critical strain or stress developed over some microstructurally significant distance. The calculated K_Ic values are found to scale roughly with the spacing of the stringer-type martensite islands associated with voids, confirming that martensite islands play an important role in reducing the toughness of the coarse-grained HAZs. These findings suggest that the formation of martensite islands should be prevented by controlling the chemical compositions and by using the proper welding conditions to enhance fracture toughness of the welded joints of the HSLA steel. Formerly Research Assistant with the Department of Materials Science and Engineering, Pohang Institute of Science and Technology     

Tempered martensite embrittlement in SAE 4340 steel

The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch (CVN) impact and compact tension plane strain fracture toughness (K _1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture, apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated with TME in the high P steel.K _IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C).     

Cr-Al钢焊接热影响区冲击韧性研究

本文采用模拟焊接热循环试验方法,测定了不同峰值温度下Cr-Al钢冲击韧性的变化,并对各峰温下的断口形貌、金相组织、晶粒度及析出物等进行了观察和鉴定。结果表明,随着峰值温度的提高,冲击韧性下降,断口形貌由韧窝状变成解理或准解理状。模拟热循环的峰温高于Ac_3时,随着峰温的提高,奥氏体晶粒明显长大,马氏体板条尺寸也相应增大,因此冲击韧性相应下降。模拟热循环的峰温低于Ac_3时,高温下马氏体板条内的碳进一步析出,基体软化,塑性提高;马氏体板条变得弯曲,碳化物聚集成球状,致使冲击韧性明显提高。     

Effects of Oxides on Tensile and Charpy Impact Properties and Fracture Toughness in Heat Affected Zones of Oxide-Containing API X80 Linepipe Steels

This study is concerned with effects of complex oxides on acicular ferrite (AF) formation, tensile and Charpy impact properties, and fracture toughness in heat affected zones (HAZs) of oxide-containing API X80 linepipe steels. Three steels were fabricated by adding Mg and O₂ to form oxides, and various HAZ microstructures were obtained by conducting HAZ simulation tests under different heat inputs. The no. of oxides increased with increasing amount of Mg and O₂, while the volume fraction of AF present in the steel HAZs increased with increasing the no. of oxides. The strengths of the HAZ specimens were generally higher than those of the base metals because of the formation of hard microstructures of bainitic ferrite and granular bainite. When the total Charpy absorbed energy was divided into the fracture initiation and propagation energies, the fracture initiation energy was maintained constant at about 75 J at room temperature, irrespective of volume fraction of AF. The fracture propagation energy rapidly increased from 75 to 150 J and saturated when the volume fraction of AF exceeded 30 pct. At 253 K (?20 °C), the total absorbed energy increased with increasing volume fraction of AF, as the cleavage fracture was changed to the ductile fracture when the volume fraction of AF exceeded 45 pct. Thus, 45 vol pct of AF at least was needed to improve the Charpy impact energy, which could be achieved by forming a no. of oxides. The fracture toughness increased with increasing the no. of oxides because of the increased volume fraction of AF formed around oxides. The fracture toughness did not show a visible correlation with the Charpy absorbed energy at room temperature, because toughness properties obtained from these two toughness testing methods had different significations in view of fracture mechanics.     

Correlation of the microstructure and fracture toughness of the heat-affected zones of an SA 508 steel

In this study, microstructures of a heat-affected zone (HAZ) of an SA 508 steel were identified by Mossbauer spectroscopy in conjunction with microscopic observations, and were correlated with fracture toughness. Specimens with the peak temperature raised to 1350 °C showed mostly martensite. With the peak temperature raised to 900 °C, the martensite fraction was reduced, while bainite or martensite islands were formed because of the slow cooling from the lower austenite region and the increase in the prior austenite grain size. As the martensite fraction present inside the HAZ increased, hardness and strength tended to increase, whereas fracture toughness decreased. The microstructures were not changed much from the base metal because of the minor tempering effect when it was raised to 650 °C or 700 °C. However, fracture toughness of the subcritical HAZ with the peak temperature raised to 650 °C to 700 °C was seriously reduced after postweld heat treatment (PWHT) because carbide particles were of primary importance in initiating voids. Thus, the most important microstructural factors affecting fracture toughness were the martensite fraction before PWHT and the carbide fraction after PWHT.     

Characterization of Gas Metal Arc Welded Hot Rolled DP600 Steel

Dual‐phase (DP) steels are suitable candidates for automotive applications due to their high strength and ductility. These advanced mechanical properties result from the special microstructure of the DP steel with 5～20% martensite phase in a soft ferrite matrix. However, during welding, which is an important process in automotive industry, this special microstructure is destroyed. In this research the characterization of Gas Metal Arc (GMA) welded joining zones was performed by optical microscopy and hardness mapping. Tensile tests were also performed keeping the welded portion in the gauge length. Scanning Electron Microscopy (SEM) was used for the fracture investigation. From the characterization and tensile tests, the soften zones were found, which are caused by the tempered martensite and larger ferrite grain size than that in base metal. Furthermore, GMA welding make a large Heat Affected Zone (HAZ).     

Microstructure,Composition, and Impact Toughness Across the Fusion Line of High-Strength Bainitic Steel Weldments

This paper analyzed the evolution of microstructure, composition, and impact toughness across the fusion line of high-strength bainitic steel weldments with different heat inputs. The main purpose was to develop a convenient method to evaluate the HAZ toughness quickly. The compositions of HAZ were insensitive to higher contents of alloy elements (e.g., Ni, Mo) in the weld metal because their diffusion distance is very short into the HAZ. The weld metal contained predominantly acicular ferrite at any a heat input, whereas the main microstructures in the HAZ changed from lath martensite/bainite to upper bainite with the increasing heat input. The evolution of HAZ toughness in relation to microstructural changes can be revealed clearly combined with the impact load curve and fracture morphology, although the results of impact tests do not show an obvious change with heat input because the position of Charpy V notch contains the weld metal, HAZ as well as a part of base metal. As a result, based on the bead-on-plate welding tests, the welding parameter affecting the HAZ toughness can be evaluated rapidly.     

大线能量焊接用EH420海工钢生产工艺及焊接性能

河钢集团有限公司开发了利用钢液中形成TiO_x?MgO?CaO细小粒子改善焊接粗晶热影响区韧性的ITFFP技术（Improve the toughness of HAZ by forming TiO_x?MgO?CaO fine particles in steel）,成功试制生产出大线能量焊接用30 mm厚度规格（H30）和60 mm厚度规格（H60）EH420海洋工程用钢。母材力学性能试验结果表明,H30和H60试制钢屈服强度分别达到461 MPa和534 MPa,抗拉强度分别达到570 MPa和628 MPa,延伸率分别为26%和24.5%,满足EH420海洋工程用钢国家标准要求。采用Gleeble-3800型热模拟试验机对试制钢进行了200 kJ·cm?1条件下热模拟试验,并对焊接热影响区中的显微组织和?40 ℃冲击韧性进行了分析和测试。结果表明,试制钢中形成的CaO(?MgO)?Al₂O₃?TiO_x?MnS夹杂物可以有效地诱导针状铁素体析出,显著提高钢材的冲击韧性。另外,利用气电立焊设备对H30和H60试制钢分别进行了焊接线能量为247 kJ·cm?1和224 kJ·cm?1的实焊试验,结果显示,H30试制钢焊接接头表面和根部焊缝处?40 ℃冲击吸收功值≥74 J,焊接热影响区≥115 J,H60试制钢焊接接头表面和根部焊缝处?40 ℃冲击吸收功值≥91 J,焊接热影响区≥75 J,焊接接头的冲击性能远高于国家标准值42 J。      

Development of High Deformation X70 Pipeline Steel Plates in NISCO

A series of trial tests for high deformation (HD) X70 pipeline steel plates were performed in NISCO,and the technical routes as thermal mechanical controlled rolling process (TMCP),TMCP + Quenching (Q) and TMCP +Q & tempering (T) were studied systematically through the plate shape quality,properties and microstructure characters.The results show that problems as plate shape and inhomogeneous microstructures are for finish rolling at low temperature and high cooling rate after the rolling by the route of TMCP.By the route of TMCP+Q,the yield strength (YS) of the trial steels is not sufficient.By the route of TMCP+QT,the YS is enhanced,as well as good toughness and plasticity due to the martensite decomposition at low temperature tempering process,and 4 sheets of HD X70 pipeline steel plates by the route TMCP+QT with superior plate shape quality,microstructure and comprehensive properties were successfully developed in NISCO.     

Study of welding ultra‐fine grained ferrite steel by CO2 laser

Ultra‐fine grained ferrite steels have higher strength and better toughness than the normal ferrite steels because of their micrometer or sub‐micrometer sized grains. In this paper the ultra‐fine grained steel SS400 is welded by CO₂ laser. The shape of weld, cooling rate of HAZ, width of HAZ, microstructures and mechanical properties of the joint are discussed. Experimental results indicate that laser beam welding can produce weld with a large ratio of depth to width. The cooling rate of HAZ of laser beam welding is fast, the growth of prior austenite grains of HAZ is limited, and the width of weld and HAZ is narrow. The microstructures of weld metal and coarse‐grained HAZ of laser beam welding mainly consist of B_L + M (small amount). With proper laser power and welding speed, good comprehensive mechanical properties can be acquired. The toughness of weld metal and coarse‐grained HAZ are higher than that of base metal. There is no softened zone after laser beam welding. The tensile strength of a welded joint is higher than that of base metal. The welded joint has good bending ductility.     

Cleavage initiation in the intercritically reheated coarse-grained heat-affected zone: Part I. Fractographic evidence

Tensile, crack opening displacement (COD), blunt notch, and Charpy impact tests were used to investigate cleavage initiation in the intercritically reheated coarse-grained heat-affected zone (IC CG HAZ) of three steels. The steels were chosen to provide different distributions and morphologies of MA (high-carbon martensite with some retained austenite) particles within the IC CG HAZ structure. Observation of minimum impact toughness values for the IC CG HAZ was found to be associated with a particular microstructure containing a near-connected grain boundary network of blocky MA particles, the MA particles being significantly harder than the internal grain microstructure. The initiation mechanism for this structure was determined to be from a combination of an overlap of residual transformational induced stress fields, due to the formation of the MA particles, between two closely spaced particles and stress concentration effects resulting from debonding of the particles.     

Effect of Inclusions Containing Ti,Mg on Microstructure and Performance of HAZ in Low Carbon Steel

In order to study the effects of inclusions containing titanium,zirconium and magnesium on the impact toughness of heat affected zone (HAZ) of steel,two low carbon steels deoxidized by titanium,zirconium and magnesium were obtained.After some treatments,the inclusion characteristics (size,morphology and chemistry) analysis,Charpy-type test,microstructure and the fracture observation were carried out on the prepared specimens.The following results were found.The inclusions containing Mg can keep more stable in HAZ.The inclusion containing Mg can enhance the impact toughness of HAZ,and the impacting energy reaches 249J at 253K when simulated input energy is 150kJ/mm.The inclusions with diameter smaller than 1.5 μm play a key role of pinning effect.     

Effect of non-metallic inclusions on fracture toughness of tool steels

The aim of these investigations was first of all to evaluate the fracture toughness (K_lc) changes of the hot-work tool steels depending on the non-metallic inclusions (NMI) volume fraction (melting technology). The tests were carried out on two types of the hot-work tool steels, i. e. H13 and H11 according to AISI. As a result of these investigations, supplemented by the detailed fractographic analysis, it has been revealed that uniform arrangement of NMI in the structure can be considered as harmless for the fracture toughness of tool steels. At high steel hardness values, the NMI, because of their action with a very small plastic strain zone, can be treated as natural obstacles in the crack propagation. At low hardness values of tool steels, achieved as a result of tempering at high temperatures, the role of NMI in the process of crack formation of these steels is limited by carbides precipitated from martensite. The micro-voids are formed round these carbides, which, connecting earlier than the voids formed round NMI, set the path of cracking and determine the steel fracture toughness.     

Correlation of microstructure and tempered martensite embrittlement in two 4340 steels

This study is concerned with a correlation between the microstructure and fracture behavior of two AISI 4340 steels which were vacuum induction melted and then deoxidized with aluminum and titanium additions. This allowed a comparison between microstructures that underwent large increases in grain size and those that did not. When the steels were tempered at 350°C,K _Ic and Charpy impact energy plots showed troughs which indicated tempered martensite embrittlement (TME). The TME results of plane strain fracture toughness are interpreted using a simple ductile fracture initiation model based on large strain deformation fields ahead of cracks, suggesting thatK _Icscales roughly with the square root of the spacing of cementite particles precipitated during the tempering treatment. The trough in Charpy impact energy is found to coincide well with the amount of intergranular fracture and the effect of segregation of phosphorus on the austenite grain boundaries. In addition, cementite particles are of primary importance in initiating the intergranular cracks and, consequently, reducing the Charpy energy. These findings suggest that TME in the two 4340 steels studied can be explained quantitatively using different fracture models.     

Static and dynamic fracture toughness of a medium carbon microalloyed steel of three different microstructures

A multiphase ferrite-bainite-martensite (F-B-M) microstructure was developed in an automotive grade V-bearing medium carbon microalloyed steel, 38MnSiVS5. It was characterized using optical, scanning, and transmission electron microscopy. The tensile, Charpy impact, and static and dynamic fracture toughness behaviors were evaluated. The results are compared with those of ferrite-pearlite (F-P) and tempered martensite (T-M) microstructures of the same steel. Although the tensile properties of the multiphase microstructures were superior, the Charpy impact and static and dynamic fracture toughness properties were inferior compared with those of the other two microstructures. The F-P condition displayed the highest plane strain fracture toughness value (K_IC), while the T-M condition was characterized by the highest dynamic fracture toughness (conditional) value (K_IDQ). The Charpy impact energy of the T-M condition was greater than that for the other two conditions. An examination of the surfaces of fractured samples revealed predominant ductile crack growth in the F-P microstructure and a mixed mode (ductile and brittle) crack growth in the T-M and the F-B-M microstructures. Although the Charpy impact energy, plane fracture toughness (K_IC), and conditional dynamic fracture toughness (K_IDQ) of the multiphase microstructure were inferior to those of the T-M and the F-P microstructures, the toughness properties were comparable to those of medium carbon low alloy steels having bainite-martensite (AISI 4340) or tempered martensite microstructures.     

High Performance Structural Steels for Shipbuilding and Offshore Structures

Recently,there have been the increase of ship size and the development of oil and gas in arctic region.These trends have led to the requirements such as high strength,good toughness at low temperature and good weldability.The high performance structural steels for shipbuilding and arctic offshore structure have been developed by our own micro-alloying and TMCP technologies.M-A constituent was precisely controlled in the both HAZ and base metal to get high toughness at low temperature.Also,the grain growth of austenite at HAZ was effectively suppressed by thermally stable TiN particles,leading to a good HAZ toughness.On the other hand,there has been the key issue of crack arrestability in large size container ship.The effect of joint design on crack arrestability was investigated to prevent a catastrophic failure along the block joint of hatch side coaming.A brittle crack arrest technique was developed without block joint shift,using an arrest weld in the end of hatch side coaming weld line.     

Effects of Zr on acicular ferrite formation in HAZ of Ti-bearing low carbon steels with high heat input of weld thermal simulations

To research the effect of Zr addition on inhibiting austenite grain growth of Ti-bearing low carbon steels,two steels with different Zr contents were prepared using a laboratory vacuum induction furnace. The performance of HAZ under weld thermal simulations was investigated. The impact toughness,microstructure and the second-phase particle performance of HAZ under weld thermal simulations were investigated. The HAZ toughness was improved from 13 J to 87 J by addition of 0. 010 % Zr into the steel,with the fracture mechanism changing from cleavage fracture to toughness fracture,which was mainly attributed to the second-phase particles that were potent to nucleate acicular ferrite in HAZ during welding. It was concluded that the second-phase particles TiO x + MnS,ZrO 2 + MnS or TiO x + ZrO 2 + MnS were nucleated on ZrO 2 or TiO x ( x =1. 5,2) . This method can be applied to grain refinement by promoting the acicular ferrite formation and growth during large-scale welding,as in the cases of thick steel plates requiring higher heat inputs during welding.     

The effect of retained austenite on the fracture toughness of high-speed steels

The aim of this work was to find the quantitative dependences between fracture toughness K_lc and the volume fraction of retained austenite in the matrix of quenched high-speed steels. The tests were carried out on three model alloys of a different content quotient of Mo: W which, after quenching, were gradually supercooled up to ? 196°C and then tempered at 450°C. Also the measurements of the content of retained austenite in the vicinity of the surface of a sample fracture were carried out. It was determined that after tempering at 450°C the fracture toughness of the matrix of high-speed steels is directly proportional to the content of retained austenite in it. Every 1 % by volume of retained austenite increases the fracture toughness K_lc of the matrix by about 5%, despite the fact that most probably it is completely transformed into fresh martensite in front of a propagating crack. Higher fracture toughness of the matrix of high-speed steels rich in molybdenum should be explained exlusively by a larger content of retained austenite. Transformations in the martensitic part of the matrix of the alloys richer in molybdenum clearly reduce the advantageous effect of retained austenite on this steel feature.     

热输入对DP600激光焊组织和力学性能的影响

摘要：为了研究DP600钢的焊接性能,采用5种不同的激光焊接工艺进行焊接试验。结果表明,焊接接头表面成形质量良好,随着热输入的增加,上下熔宽逐渐增大;熔融区均为板条状马氏体组织,当热输入高于33J/mm时热影响区组织为马氏体、铁素体和少量的回火马氏体;当热输入低于33J/mm时,热影响区组织为马氏体和铁素体。在低热输入条件下,回火时间很短,马氏体未发生分解;在高的热输入条件下,回火时间较长,马氏体分解显著,热影响区中出现M3C型碳化物,碳化物形貌以球状和片状为主。从熔融区到母材,显微硬度值逐渐降低;焊接接头静态拉伸失效位置均在母材,拉伸断口为韧性断口,DP600钢激光焊接接头不存在软化现象。