Microstructure model for the heat-affected zone of X80 linepipe steel

An important aspect of the integrity of oil and gas pipelines is the heat-affected zone (HAZ) of girth welds where the microstructure of the as-hot rolled steel is altered with potentially adverse effects on the HAZ properties. Therefore, it is critical to evaluate the HAZ microstructure for different welding scenarios. Here, an integrated microstructure evolution model is proposed and applied to the HAZ of an X80 linepipe steel. The model considers dissolution of Nb-rich precipitates, austenite grain growth and austenite decomposition into ferrite and bainite. Microstructure maps showing the fraction of transformation products as a function of distance from the fusion line are obtained and used to compare the effect of different welding procedures on the HAZ microstructure.     

Effects of nitrogen content and weld cooling time on the simulated heat-affected zone toughness in a Ti-containing steel

An increase of nitrogen content in a 0.02 wt% Ti-containing carbon-manganese steel resulted in a low coarsening rate of TiN particles in the heat-affected zone (HAZ), which led to an accelerated ferrite transformation instead of ferrite side plates during weld cooling cycle. The mixed microstructure of ferrite side plate, acicular ferrite and grain boundary polygonal ferrite in the simulated HAZ produced higher toughness. However, the increase of nitrogen content gradually increased the free nitrogen content in the HAZ and deteriorated HAZ toughness. Impact energy of the simulated HAZ (with Δt_8/5 ∼60 s) at –20 °C deteriorated by about 97 J per 0.001 wt% free nitrogen, in the free nitrogen range from 0.0009 wt% to 0.0034 wt%, even though the HAZ has the tough mixed microstructure. Cooling time after welding influenced the HAZ microstructure and toughness as well, and maximum toughness was obtained when cooling produced the tough mixed microstructure. Therefore, for a high HAZ toughness, both nitrogen content and cooling time should be controlled to obtain the tough mixed microstructure and to keep the free nitrogen content low. The optimal nitrogen content and cooling time from 800 °C to 500 °C were 0.006 wt% and between 60 s and 100 s, respectively, in this experiment.     

Comparative study of heat-affected zone with weld and base material after post-weld heat treatment of HSLA steel using ball indentation technique

Ball indentation technique (BIT) is employed to study the effect of post-weld heat treatment on the mechanical properties of a high strength low alloy (HSLA) steel and subsequently on the quality of the weldment. Well-defined load-deflection curves and corresponding true stress–strain curves for different zones (base, HAZ, weld) of sectioned sample (top and middle) and their validation with mechanical properties obtained by conventional method established the effectiveness of the present ball indentation (BI) set up. Investigations on microstructure of all the zones have been carried out to find out a correlation with the obtained mechanical properties. Evaluation of the mechanical properties of materials through BIT could characterize the heat-affected zone in weld HSLA steel.     

Effect of the structural state on cyclic cracking resistance of quenched and tempered structural steel

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 25, No. 2, pp. 57–61, March–April, 1989.     

Effect of joint design on ballistic performance of quenched and tempered steel welded joints

A study was carried out to evaluate the effect of joint design on ballistic performance of armour grade quenched and tempered steel welded joints. Equal double Vee and unequal double Vee joint configuration were considered in this study. Targets were fabricated using 4 mm thick tungsten carbide hardfaced middle layer; above and below which austenitic stainless steel layers were deposited on both sides of the hardfaced interlayer in both joint configurations. Shielded metal arc welding process was used to deposit for all layers. The fabricated targets were evaluated for its ballistic performance and the results were compared in terms of depth of penetration on weld metal. From the ballistic test results, it was observed that both the targets successfully stopped the bullet penetration at weld center line. Of the two targets, the target made with unequal double Vee joint configuration offered maximum resistance to the bullet penetration at weld metal location without any bulge at the rear side. The higher volume of austenitic stainless steel front layer and the presence of hardfaced interlayer after some depth of soft austenitic stainless steel front layer is the primary reason for the superior ballistic performance of this joint.     

Adiabatic Shear in annealed and shock-hardened iron and in quenched and tempered 4340 steel

Adiabatic shear localization is a catastrophic failure mechanism which can occur in ductile metals under high strain rate loading. This mechanism is driven by a thermal instability process in which rapid temperature rise due to plastic work couples with thermal softening to cause uniform deformation to collapse into narrow bands of intense shear within which material ductility is exhausted. Adiabatic shear localization is studied in three ferrous metals: annealed Armco and as-received Remco iron, both of which are high purity alpha iron; shock-hardened Remco iron; and 4340 steel quenched and tempered to a range of hardness levels. Using a compressive split-Hopkinson bar, punching-shear experiments were performed at room and elevated initial temperatures at shear strain rates of up to 45000 s^–1. Optical and scanning electron microscopy was performed on the deformed shear specimens to determine the extent of shear localization and mode of failure. Experimental evidence showed that the tempered 4340 steels were susceptible to localization through adiabatic shear banding; however, as-received and shock-hardened Remco iron and annealed Armco iron were not. Finite element simulations of the experiments were performed utilizing a user material subroutine developed as part of this research. This constitutive routine incorporates two adiabatic shear failure criteria, namely (i) maximum shear stress with a minimum critical shear strain rate and (ii) flow localization. These criteria proved to be capable of predicting the onset of an instability; however, the deformation which follows the instability was not predicted well.     

Evaluation of the subsurface microstructure of quenched and tempered carbon steel by ultrasonic backscatter

The behaviour of 5 MHz ultrasound backscattered at the Rayleigh angle from waterimmersed specimens of En3A, En9 and En25 steel has been related to microstructural features induced by heat treatment. The range of phase structures considered is a substantial extension on what has been described in the previous literature. In the case of the En25 samples, which all had the same mean grain size, the backscattered energy decreased systematically with increasing hardness, within the standard deviation of the energy backscattered from different regions of the same sample. When all samples of the three types of steel were compared at constant grain size in the Rayleigh scattering region, the backscattered energy was found to vary systematically with phase structure in the decreasing sequence: 80% pearlite-20% ferrite > bainite > tempered martensite > 80%ferrite-20%pearlite > acicular ferrite > martensite. The intensity of backscattering can be changed by up to 20 dB by heat treatment, with a standard deviation of ∼ 1.5 dB for repeat measurements at the same sample point, while the standard deviation for thesignal intensity arising from different regions of the same specimen surface varied between 2 and 4 dB. It appears that the backscatter technique can readily distinguish between different phase structures as produced, for example, by oven cooling and air cooling, and it is worth noting that the data were obtained using a standard, relatively inexpensive, ultrasonic flaw detector.     

Manual metal arc weld modelling: Part 1 Effect of process parameters on dimensions of weld bead and heat-affected zone

Abstract

The properties of a weldment are determined largely by the size and distribution of microstructural regions within the weld metal and heat-affected zone (HAZ). It has been appreciated for some time that these properties may be controlled by adjusting the parameters of the welding process, although until recently this was done mainly qualitatively. Means of achieving more quantitative control are now beginning to be applied. In this paper measurements of the sizes of the weld bead and HAZ are presented for single manual metal arc weld beads. The process variables investigated were electrode type, gauge size, welding position, polarity, welding current, preheat, and welding velocity. Functional relationships between the process variables and the size of the weld bead and HAZ are determined. The generality of these relationships is examined by analysing measurements made previously on other materials. The results are discussed in terms of the theory of the welding process and, as shown in two accompanying papers, they can be used to develop models of multipass manual metal arc welding on which a practical welding procedure may be based.

MST/193a     

Precipitate alterations in the heat-affected zone of a Grade 100 microalloyed steel

Microalloy precipitate alterations (particularly dissolution) in the heat-affected zone (HAZ) of a Grade 100 steel, microalloyed by titanium, niobium, and vanadium and produced in the form of a plate with a thickness of 8 mm, was examined both theoretically and experimentally. For theoretical analysis of precipitate dissolution, pairs of effective peak temperature and holding time were extracted from the thermal cycles of welding, and were superimposed on the Ashby and Easterling non-equilibrium solubility curves for different fractions of precipitate dissolution. Intersections between the effective T–t curves and the non-equilibrium solubility curves gave critical pairs of effective peak temperature and holding time for dissolution of different fractions of a precipitate, which resulted in the establishment of precipitate dissolution profiles in the HAZ. Experimental analysis of precipitate alterations was carried out using carbon extraction replicas in a transmission electron microscope. The theoretical analyses were in agreement with experimental results, showing that it is the dissolution of small Nb-rich particles that paves the way to grain growth in the coarse-grained HAZ. Reprecipitation was generally suppressed in the low heat-input weld sample. There was some reprecipitation in the higher heat-input weld samples. Coarsening of TiN did not occur in the HAZ, due to the large size of these particles in the steel examined.     

Effect of back-stress on cyclic stress-strain behaviour of a quenched and tempered low alloy steel

Low cycle fatigue was considered in relation to back-stress hardening. Cyclic stress-strain behaviours under controlling strain and stress conditions were investigated for a quenched and tempered low alloy steel which contained cementite particles. The cyclic stress-strain states obtained by the two prescribed tests were uniquely described using a parameter which reflected the cumulative reversible plastic work associated with the back-stress hardening. It was suggested that the effect of back-stress hardening on cyclic deformation should appear directly on the cyclic stress-strain curves defined as the loci of the tips of stable hysteresis loops. The initial slopes of the cyclic stress-strain curves for several steels were demonstrated to coincide with the theoretical work-hardening rates calculated on the basis of back-stress hardening due to the included carbide particles. Finally, the Manson-Coffin law was explained from the view that the surface damage would progress in parallel with the structure change in the bulk according to the persistency of slips resulting from the reversible back-stress hardening.     

Effects of nitriding on fatigue strength of quenched and tempered steel: role of interstitial nitrogen

Abstract

This paper describes an investigation into the effects of nitriding, denitriding, and renitriding, on the rotating bending fatigue strength of notched specimens of the quenched and tempered steels 24CrMo13 and 42CrMo4. The effects of the different treatments were characterised by microstructural investigation, hardness measurements, determination of the nitrogen and carbon contents in the diffusion layer, and determination of surface macrostresses. Nitriding induced a doubling of the fatigue limit. Denitriding only partly removed the effect of the nitriding treatment, because the strength increase caused by precipitation of alloying element nitrides remained unchanged, and only the effect of interstitially solved nitrogen disappeared. For nitrided and renitrided specimens an approximately linear relationship was found between the surface hardness and the fatigue strength. Denitriding caused a larger decrease in the fatigue strength than could be explained by the loss of surface hardness. The change in macrostresses from compressive to tensile also played a part. The role of interstitially dissolved nitrogen in determining the fatigue strength was found to be larger than assumed thus far.     

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.     

Towards a prediction of the hardness of the heat-affected zone of steel weldments

The prediction of the level of hardness developed in the heat-affected zone (HAZ) of steel welds is discussed. It is composed of a thermal model that predicts the cooling behaviour from input welding parameters and a material model for calculating the HAZ hardness as a result of weld cooling. Experimental investigations were carried out on eight different steel welds using three different electrodes under two different welding processes. Comparisons of the experimental results as well as the experimental results reported in the open literature, against the calculated values for both HAZ hardness and cooling time, were conducted. The results presented in this paper show good agreement between calculated and measured values of both cooling rate and hardness. The calculations can be carried out readily in small pocketsized computers. On Leave from Mechanical Engineering Department, Assiut University, Assiut, Egypt.     

Effect of acicular ferrite formation on grain refinement in the coarse-grained region of heat-affected zone

The microstructure of acicular ferrite and its formation for the grain refinement of coarse-grained region of heat-affected zone of high strength low-alloy bainite steels were studied using three-dimensional reconstruction technique. Crystallographic grain size was analyzed by means of electron backscatter diffraction. It was revealed that the microstructure in the coarse-grained region of the heat-affected zone consisted of predominantly bainite packets and a small proportion of acicular ferrite. Acicular ferrite was of lath or plate-like rather than needle or rod-like morphology. Tempering of the coarse-grained region of heat-affected zone showed that the acicular ferrite was more stable than the bainite, indicating that the acicular ferrite was formed prior to bainite. The acicular ferrite laths or plates divided the prior austenite grains into smaller and separate regions, and confining the bainite transformed at lower temperatures in the smaller regions and hence leading to the grain refinement in the coarse-grained region of the heat-affected zone.     

Multiaxial fatigue of quenched and tempered U2 steel: Testing and fatigue life prediction

Tension‐compression, tension‐tension, torsional, and 90° out‐of‐phase axial‐torsional fatigue tests were performed on a quenched and tempered U2 steel. All tests were conducted under force/torque control because macroscopic plastic strains were insignificant in the life range of interest (from 10⁴ to 2 × 10⁶ loading cycles). Stress‐based versions of the Fatemi‐Socie critical plane parameter and of the Smith‐Watson‐Topper parameter with a critical plane interpretation were evaluated using the experimental data. The Smith‐Watson‐Topper parameter was not able to correlate the test data. The Fatemi‐Socie method correlated most of the test data within factor‐of‐three boundaries. A modified Crossland invariant‐based parameter made of two interaction rules between the shear stress amplitude and the maximum hydrostatic stress, and of a definition of shear stress amplitude based on the maximum prismatic hull method, yielded fatigue life estimates in reasonable agreement with the experimental observations.