K. M. Wu

Toughness improvement by Cu addition in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels

The effect of microstructure and Cu addition in a simulated coarse-grained heat-affected zone (CGHAZ) of a high-strength low-alloy (HSLA) steel subjected to 100 kJ cm−1 heat input welding was studied. It has been observed that the primary microstructure in Cu-free HSLA steels is dominated by bainite, whereas, in Cu-bearing HSLA steels, the predominant microstructure is acicular ferrite. The acicular ferrite nucleated at intragranular complex inclusions consisting of Al and Ti oxides, covered with layer of MnS and CuS. The presence of high intensity of acicular ferrite and hard impingements between acicular ferrite laths or plates has contributed to the fine-grained and interlocked microstructure. The enhanced toughness in CGHAZ of Cu-bearing HSLA steel is attributed to the fine-grained interlocked microstructure of acicular ferrite.

In situ observation of acicular ferrite formation and grain refinement in simulated heat affected zone of high strength low alloy steel

Abstract The phase transformation from austenite to acicular ferrite in the simulated coarse grained heat affected zone of a high strength low alloy steel was investigated by means of analytical characterisation techniques such as in situ microscopy, transmission electron microscopy and electron backscattered diffraction analysis. The acicular ferrite grains nucleated on inclusions (Zr–Ti oxides) in coarse austenite grain grew in different directions and effectively partitioned coarse austenite grain into several finer and separate regions. The crystallographic grain size became small for coarse austenite grains due to the effective partitioning by acicular ferrite laths or plates.

Characteristic of martensite–austenite constituents in coarse grained heat affected zone of HSLA steel with varying Al contents

Abstract The fine microstructure of martensite–austenite (M–A) constituents in simulated coarse grained heat affected zone (HAZ) of high strength low alloy steel with varying aluminium content (0·038 and 0·070 wt-%) at 100 kJ cm−1 heat input welding was investigated. The result shows that M–A constituents with 0·038%Al consisted of lath martensite and retained austenite. The retained austenite was distributed along the martensite lath. Whereas, the M–A constituents with 0·070%Al consisted of lath martensite and retained austenite, as well as a small amount of twinned martensite. The amount of retained austenite in M–A constituents with 0·070%Al was becoming higher slightly than that with 0·038%Al. Accordingly, the volume fraction of M–A constituents was reduced with 0·070%Al. Appropriate aluminium addition could decrease not only the area fraction but also the size of M–A constituents, which are beneficial for improving the toughness of HAZ.

Effect of fast cooling process on microstructure and toughness of heat affected zone in high strength pipeline steel X120

Abstract A process for improving the heat affected zone (HAZ) toughness by fast cooling during welding was developed. The holding time at high temperature above Ac3 and the cooling time from 800 to 500°C were decreased when the welding thermal cycle was experienced a fast cooling process. The microstructures in the HAZ with varied thermal cycles were examined by optical and scanning electron microscopes. The shorten holding time led to thinner HAZ width and finer austenite grains in the fusion line and coarse grained HAZ, while the decreased cooling time from 800 to 500°C resulted in finer bainitic ferrite in the HAZ. Martensite–austenite constituents in the fusion line were reduced in amount and size and predominantly changed into film-like shape. Experimental results indicated that the fast cooling process played an important role in improving the HAZ toughness of pipeline steel X120.

Improvement of impact toughness of simulated heat affected zone by addition of aluminium

Abstract The effect of aluminium content (0·023, 0·038 and 0·070 wt-%) on microstructure and impact toughness of simulated coarse grained heat affected zone (CGHAZ) of high strength low alloy steels with different heat inputs (20, 100 and 200 kJ cm−1) was investigated. The microstructure of simulated CGHAZ consisted of predominantly granular bainite. The martensite–austenite constituents became finer and its volume fraction decreased with increasing aluminium content, irrespective of heat input level. The impact toughness of the simulated CGHAZ improved remarkably with increasing aluminium content even at high heat input of 200 kJ cm−1. It was attributed to the reduction in volume fraction of martensite–austenite constituents and the refined martensite–austenite constituents by addition of appropriate aluminium.

Effect of retained austenite on wear resistance of nanostructured dual phase steels

Nanostructured super bainitic and quenching–partitioning (Q&P) martensitic steels with a significant amount of retained austenite obtained by low temperature bainitic transformation and Q&P respectively were studied to explore the effect of retained austenite on stirring wear resistance. The results suggest that the Q&P martensitic steel significantly enhanced the hardness of the worn surface (from 674 to 762 HV1) and increased the thickness of the deformed layer (∼3.3 μm), compared to the nanostructured bainitic steel. The underlying reason is that the Q&P martensitic steel has a higher stability of retained austenite thereby providing a superior transformation induced plasticity effect to increase surface hardness and reduce wear rate during the wear process.

Effect of fast cooling on microstructure and toughness of heat affected zone in high strength offshore steel

Abstract E690 is a newly developed high strength high toughness steel for offshore structures. The effect of different cooling modes on the microstructure and toughness of the heat affected zone in E690 weldment were investigated in this work. The outcome of microstructural examinations and mechanical tests showed that fast cooling immediately after submerged welding can reduce the width of heat affected and coarse grained zones, as well as improving the low temperature impact toughness. It was also shown that reduction in the grain size of the parent austenite and having lower amounts of retained austenite within bainitic structure are the main causes of the observed improvement.

Microstructural characteristics and impact toughness in YS690MPa steel weld metal for offshore structures

The fine-grained mixed microstructure of acicular ferrite (AF) and bainite in YS690MPa steel weld metal contributes to attain high-impact toughness. The morphology and evolutionary mechanism of fine-grained mixed microstructure in this weld metal were investigated. Single or multiple AF grains were nucleated on complex inclusions by forming Mn-depleted zones, where Mn spontaneously diffused into Ti oxide inclusions due to the cation vacancies. It is in good agreement with the theoretical calculation by first principle. The bainite nucleated on austenite grain boundary and then assisted the pre-formed AF to partition the austenite grain into small and separate regions. Furthermore, the later formed ferrite nucleated on the broad surface of pre-formed ferrite plates and grew in those small regions with limited grain size. All of them resulted in the formation of fine-grained mixed microstructure, which provided excellent impact toughness in this weld metal with dimples and quasi-cleavage fracture surface combination.

In-situ microscopy study of grain refinement in the simulated heat-affected zone of high-strength low-alloy steel by TiN particle

ABSTRACT High-strength low-alloy steels subjected to high heat input welding are susceptible to failure due to low toughness caused by grain coarsening. The effect of TiN on grain refinement in the simulated heat-affected zone (HAZ) was investigated. Because of small amount of Ti addition, abundant dispersed nanoscale TiN precipitates were formed. The TiN precipitates tended to be stable at high temperature and effectively retarded the austenite grain growth by refining the grain size during thermal cycle. Furthermore, the TiN also covered on the surface of Al–Ti complex oxide with MnS and caused low interface energy with ferrite. The acicular ferrite grains nucleated on complex inclusion in austenite grains at intermediate temperature and induced the austenite grain transform to the fine-grained mixed microstructure of acicular ferrite and bainite. The crystallographic grain size became small in the simulated HAZ due to the effective pinning effect and acicular ferrite formation.

Effect of Zr–Ti deoxidisation on the hydrogen-induced cracking of X65 pipeline steels

The effect of Zr–Ti combined deoxidisation, compared with the traditional Al deoxidisation, on inclusion and microstructure in X65 pipeline steels was investigated by means of analytical characterisation techniques such as non-aqueous solution electrolysis method, optical and electron microscopy in conjunction with energy dispersive X-ray spectroscopy and hydrogen-induced cracking test. Large inclusions as well as elongated MnS particles were observed in the conventional Al-deoxidised steel. However, the type and size of inclusions were efficiently controlled in the Zr–Ti-deoxidised steel, in which MnS particles were spheroidised and evenly dispersed in the matrix. Hydrogen-induced cracking test results showed that centre segregation was the main factor accountable for hydrogen-induced cracking in X65 pipeline steels. The Zr–Ti-deoxidised X65 pipeline steel revealed better HIC-resistance performance.