Effects of alloying elements on fracture toughness in the transition temperature region of base metals and simulated heat-affected zones of Mn-Mo-Ni low-alloy steels

This study is concerned with the effects of alloying elements on fracture toughness in the transition temperature region of base metals and heat-affected zones (HAZs) of Mn-Mo-Ni low-alloy steels. Three kinds of steels whose compositions were varied from the composition specification of SA 508 steel (grade 3) were fabricated by vacuum-induction melting and heat treatment, and their fracture toughness was examined using an ASTM E1921 standard test method. In the steels that have decreased C and increased Mo and Ni content, the number of fine M₂C carbides was greatly increased and the number of coarse M₃C carbides was decreased, thereby leading to the simultaneous improvement of tensile properties and fracture toughness. Brittle martensite-austenite (M-A) constituents were also formed in these steels during cooling, but did not deteriorate fracture toughness because they were decomposed to ferrite and fine carbides after tempering. Their simulated HAZs also had sufficient impact toughness after postweld heat treatment. These findings indicated that the reduction in C content to inhibit the formation of coarse cementite and to improve toughness and the increase in Mo and Ni to prevent the reduction in hardenability and to precipitate fine M₂C carbides were useful ways to improve simultaneously the tensile and fracture properties of the HAZs as well as the base metals.     

Evaluation of the fracture toughness of hot-rolled low-alloy Ti-V plate steel

The effects of varying finish rolling temperature (FRT) and cooling rate on the mechanical properties of hot rolled plates of an experimental low-alloy Ti-V steel were studied. Fracture toughness was evaluated for various types of specimens at slow and high deformation rates. However, the transition temperatures determined by the various tests do not always correlate. Therefore, it is recommended that fracture toughness be evaluated by both static and dynamic testing. Following a low cooling rate, the best plate properties are obtained at the lowest FRT in the austenite-ferrite range, although occurrence of delaminations at this temperature may be detrimental for specific applications. Higher cooling rates produce higher strength but lower toughness than lower cooling rates in plates with the same FRT.     

Effects of microstructure on fracture toughness of a high-strength low-alloy steel

Fracture toughness and ductile-brittle transition behavior were measured for a copper-bearing HSLA steel. The value ofK _lc for cleavage failure was independent of heat treatment, whileJ _lc for ductile failure decreased monotonically with increasing strength level. With both failure modes, fracture appears to be controlled by cracking of sulfide inclusions. The decrease in ductile-failureJ _lc is caused by decreased work-hardening rates that suppress cleavage and facilitate void coalescence. Both higher austenitizing temperature and quenching rate after austenitization influence the ductile/brittle transition temperature, either through grain-size and precipitate refinement or through an increase in the resistance of the steel to shear failure. Formerly Graduate Student, The Ohio State University     

Mechanism of detrimental effects of carbon content on cleavage fracture toughness of low-alloy steel

The variation in fracture toughness of low-alloy base steels and weld steels with carbon contents of 0.08 and 0.21 wt pct was investigated using notched and precracked specimens tested at low temperatures. The attention is focused on the mechanism associated with detrimental effects on cleavage fracture toughness resulting from increasing carbon content. Analyses reveal that, in the case of constant ferrite grain sizes with increasing carbon content, the yield stress σ _y increases and the local fracture stress σ _f remains constant for notched specimens. For precracked specimens, the σ _y increases, whereas the σ _f decreases. In both cases, the ratio σ _f /σ _y decreases; this ratio is one of the principal factors inducing the deterioration in the cleavage fracture toughness of the higher carbon steels. Analyses also reveal that the critical strain for initiating a crack nucleus, which decreases with increasing carbon content and impurity elements, appears to be another principal factor that has a negative effect on the fracture toughness in both notched and precracked specimens. The results of the fracture toughness measured for weld metal with various grain sizes further support the predominant effect of grain size on the toughness of notched specimens.     

Effects of tensile prestrain on the notch toughness of low-alloy steel

Tensile prestrains of various levels were applied to blank steel specimens. Four-point bend tests of notched specimens at various temperatures revealed an appreciable drop in the notch toughness of the specimens, which experienced 3 pct tensile prestrain. Further prestrains of up to 20 pct had a negligible effect on the notch toughness despite additional increases in the yield strength. Microscopic analyses combined with finite element method (FEM) calculations revealed that the decrease in toughness resulted from a change of the critical event controlling the cleavage fracture. The increase in yield strength provided by prestraining allowed the normal tensile stress at the notch tip to exceed the local fracture stress σ _f for propagating a just-nucleated microcrack. As a result, for the coarsegrained steel with low σ _f tested presently, the critical event was changed from tensile stress-controlled propagation of a nucleated microcrack to plastic strain-controlled nucleation of the microcrack at the notch tip. A reduction of toughness was induced as a result of this. The increase in yield strength provided by decreasing the test temperature acted in the same way.     

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     

低合金钢焊接热影响区的微观组织和韧性研究进展

对钢结构而言,诸如海洋平台、船舶、桥梁、建筑和油气管线等,焊接后的性能直接决定了其服役寿命和安全性,重要性不言而喻.在针对焊接相关问题的研究中,焊接热影响区的韧性提升一直是重点和难点.焊接热影响区会经历高达1400℃的高温,从而形成粗大的奥氏体晶粒,如果焊接参数控制不当,不能通过后续冷却过程中的相变细化组织,就会造成韧性的降低.而多道次焊接的情况更为复杂,前一道次形成的粗晶区还会在后续焊接过程中经历二次热循环,从而形成链状M-A,造成韧性的急剧下降.本文旨在对一些现有焊接热影响区的相关研究结果进行总结,探讨母材的成分、第二相及焊接工艺等因素对热影响区微观组织和性能的影响,为低温环境服役的大型钢结构的焊接性能改善提供一些设计思路.     

Effect of decreased hot-rolling reduction treatment on fracture toughness of low-alloy structural steels

Commercial low-alloy structural steels, 0.45 pct C (AISI 1045 grade), 0.40 pct C-Cr-Mo (AISI 4140 grade), and 0.40 pct C-Ni-Cr-Mo (AISI 4340 grade), have been studied to determine the effect of the decreased hot-rolling reduction treatment (DHRRT) from 98 to 80 pct on fracture toughness of quenched and highly tempered low-alloy structural steels. The significant conclusions are as follows: (1) the sulfide inclusions were modified through the DHRRT from a stringer (mean aspect ratio: 16.5 to 17.6) to an ellipse (mean aspect ratio: 3.8 to 4.5), independent of the steels studied; (2) the DHRRT significantly improvedJ _Ic in the long-transverse and shorttransverse orientations, independent of the steels studied; and (3) the shelf energy in the Charpy V-notch impact test is also greatly improved by the DHRRT, independent of testing orientation and steels studied; however, (4) the ductile-to-brittle transition temperature was only slightly affected by the DHRRT. The beneficial effect on theJ _Ic is briefly discussed in terms of a crack extension model involving the formation of voids at the inclusion sites and their growth and eventual linking up through the rupture of the intervening ligaments by local shear.     

Mixed mode I/III fracture toughness of an experimental rotor steel

Mixed mode I/III toughness tests were performed on a Ni-Cr-Mo-V rotor steel that exhibited very high mode I toughness. The minimum toughness occurred under mixed mode loading conditions. The influence of mode III loading on total toughness is analyzed in terms of incompatibility effects at particles ahead of the crack tip.     

Combined mode I-mode III fracture of a high strength low-alloy steel

Special formulations of theJ integral were used to quantify the combined mode I-mode III fracture behavior of ASTM A710 Grade A steel, a tough material which displays elastic-plastic behavior at room temperature. Experimental fracture initiation loci for two heat-treated conditions were linear inJ _i-J_iii space, agreeing with analytical predictions based on linear elasticity. As commonly observed in mode I fracture behavior, large tearing modulus values accompanied largeJ values for mode I and mode III components, individually as well as for totalJ values. There was a pronounced tendency toward antiplane shear flow in the modified compact tension specimens tested. This may result from the concentration of mode III shear strains in the plane of the propagating crack as predicted by slip line theory. Shear fractures with the crack propagating at inclined angles to both load line and plate free surfaces are favored energetically over pure mode I cracks even though the total surface area formed by the latter is much smaller.     

Combined mode I-mode III fracture of a high strength low-alloy steel

Special formulations of theJ integral were used to quantify the combined mode I-mode III fracture behavior of ASTM A710 Grade A steel, a tough material which displays elastic-plastic behavior at room temperature. Experimental fracture initiation loci for two heat-treated conditions were linear inJ _i-J_iii space, agreeing with analytical predictions based on linear elasticity. As commonly observed in mode I fracture behavior, large tearing modulus values accompanied largeJ values for mode I and mode III components, individually as well as for totalJ values. There was a pronounced tendency toward antiplane shear flow in the modified compact tension specimens tested. This may result from the concentration of mode III shear strains in the plane of the propagating crack as predicted by slip line theory. Shear fractures with the crack propagating at inclined angles to both load line and plate free surfaces are favored energetically over pure mode I cracks even though the total surface area formed by the latter is much smaller. formerly Graduate Research Assistant, The Ohio State University     

Structure, properties, and fracture of a low-alloy steel in the submicrocrystalline state

Experimental data on the structure and properties of 09G2S low-alloy steel with a submicrocrystalline (SMC) structure has been obtained; the structure was formed under various regimes of severe plastic deformation by equal-channel angular pressing. The possibility of increasing the strength characteristics and cold resistance of a ferrite-pearlite steel with a SMC structure is shown. The factors responsible for the increase in the brittle fracture resistance of an SMC material are found by studying fracture micromechanisms and fracture-surface micromorphological parameters as functions of the structure.     

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.     

Correlation of microstructure and fracture toughness in three high-speed steel rolls

The objective of this study is to clarify the fracture characteristics of high-speed steel (HSS) rolls in terms of microstructural factors such as matrix phase and primary carbide particles. Three HSS rolls with different chromium contents were fabricated by centrifugal casting, and the effect of the chromium addition was investigated through microstructural analysis, fracture-mechanism study, and toughness measurement. The hard and brittle primary carbides, as well as the eutectic carbides (ledeburites), were segregated in the intercellular regions and dominated overall properties. Observation of the fracture process revealed that these primary carbides cleaved first to form microcracks at low stress-intensity factor levels and that the microcracks then readily propagated along the intercellular networks. The addition of chromium to a certain level yielded microstructural modification, including the homogeneous distribution of primary carbides, thereby leading to enhancement of fracture toughness of the HSS rolls.     

Relationship between grain size and fracture toughness of tool steel

The aim of this study was to examine the effect of grain size, regulated by means of austenitizing temperature, on the process of hot-work tool steel cracking. Fracture toughness (K_lc) of this steel depended on the ratio of the average linear intercept of grain (L?) and the diameter of the plastic strain zone formed ahead of the propagating crack (d_y). Investigations were supplemented by the fractographic analysis. The dependence of the K_lc one the ratio L?:d_y has been confirmed. In the subcritical range, when L?:d_y<1, the grain growth causes rapid decrease of fracture toughness. In an overcritical range, when L?:d_y>1, grain growth resulting from the increase of austenitizing temperature causes stoppage of the decrease of fracture toughness and can even lead to increase. Thus, tools requiring the highest hardness can be quenched from possibly high temperatures without fear of any disadvantageous influence upon the toughness of even a strong grain growth. In the critical range, when L?:d_y≈1, fracture toughness of tool steel reached minimum.