Coarse-grained Heat-affected Zone Research Articles

Influence of ICCGHAZ on the Low-Temperature Toughness in HAZ of Heavy-Wall X80 Pipeline Steel

Low-temperature embrittlement in the heat-affected zone (HAZ) of heavy-wall X80 weld joints is a primary challenge for arctic oil & gas exploitation. In this paper, the influence of intercritically reheated coarse-grained HAZ (ICCGHAZ) on the low-temperature toughness of the weld joint in a 22 mm thick X80 spiral submerged arc welded pipe was studied through instrumented Charpy V-notch impact test at −80~20 °C and corresponding fracture surface characterization. The results indicated that the influence of ICCGHAZ on the overall toughness of the weld joint is related to temperature. At temperatures below −45 °C, individual and tiny martensite-austenite (MA) constituent debonding can trigger cleavage fracture–which was proved to be nucleation-controlled–and the probability of embrittlement of the ICCGHAZ increases. At temperatures higher than −45 °C, only relatively large or closely distributed MA constituent in ICCGHAZ satisfies the conditions to trigger propagation-controlled cleavage fractures, and the influence of ICCGHAZ on the overall toughness is not remarkable.

Effect of Heat-Input on Microstructure and Toughness of CGHAZ in a High-Nb-Content Microalloyed HSLA Steel.

The effect of various heat inputs on the microstructure and impact toughness of the simulated coarse-grained heat-affected zone (CGHAS) of a niobium microalloyed (0.14 wt.%) low-carbon steel was studied. The results showed that higher impact toughness was achieved at a low heat input of 20 kJ/cm, which resulted from the formation of acicular ferrite laths/plates. They sectioned large prior austenite grains into many smaller regions, resulting in smaller crystallographic grains and high-angle grain boundaries. Conversely, when specimens were simulated with larger heat-inputs (100, 200 kJ/cm), the microstructure of the CGHAZ was predominantly composed of granular bainite plus massive MA constituents, thus impairing the impact toughness.

Effect of Niobium Content on the Microstructure and Mechanical Properties of Simulated Coarse-Grained Heat-Affected Zone (CGHAZ) of High-Strength Low-Alloy (HSLA) Steels.

The effect of Nb-content and heat input rate on the mechanical properties and microstructure of simulated coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steel (HSLA) was investigated. While using a low heat input (20 kJ/cm), the toughness of simulated CGHAZ was improved by increasing the Nb-content. The maximum toughness was obtained when the Nb-content was 0.110 wt.% and the heat input was 20 kJ/cm. The samples made at this condition had fine martensite/austenite (M/A-constituent), acicular ferrite and refined austenite grains. As the heat input was increased to 200 kJ/cm, the toughness of simulated CGHAZ was significantly decreased irrespective of the Nb-content because of the formation of coarse austenite grains, low angle grain boundaries, and massive M/A-constituents.

Correlation between Microstructure and Mechanical Properties of Welded Joint of X70 Submarine Pipeline Steel with Heavy Wall Thickness

This paper aims to study the relationship between the microstructure and the mechanical properties of X70 submarine pipeline steel with 40.5 mm thickness. The microstructure was examined by using optical microscopy, scanning electron microscopy and an electron backscattered diffractometer, while the mechanical properties were examined by using a hardness test, a tensile test, a Charpy impact test and a drop weight tear test (DWTT), respectively. The results show that the base metal (BM) of the pipe has a low yield ratio of 0.83 and an excellent elongation of more than 45%. The DWTT shear area of the steel plate reaches 87%, showing excellent low-temperature toughness. The Charpy impact energy increases when the distance from the fusion line increases, and it reaches a maximum at the BM near the heat-affected zone (HAZ) due to the small martensite-austenite (MA) constituents and fine grains. The concentrated distribution of blocky/slender MA constituents along the prior austenite grain boundaries of the intercritically reheated coarse-grained HAZ and the large MA constituents are the main reasons for the deteriorating impact toughness. Delamination cracks in the DWTT fracture surface only occurred in the midthickness of a sample with a small opening width that spread about 2.1 mm perpendicular to the DWTT fracture surface and were finally arrested at the acicular ferrite clusters containing a high density of high-angle boundaries.

Effects of long-term thermal aging on the microstructure and mechanical behaviors of 16MND5/Alloy 152 dissimilar metal weld

The effect of thermal aging on microstructural evolution and mechanical behavior was investigated via advanced instrumental analysis. The scanning electron microscope (SEM) results demonstrated that the microstructure transformed from martensite into martensite/bainite mixture in the coarse-grained heat-affected zone (CGHAZ) of 16MND5. The micro-hardness in the CGHAZ has a significantly positive correlation with thermal aging time owing to the change of microstructure and the increased number of M23C6 precipitates. Moreover, the tensile strength and elongation of the dissimilar metal welds have a positive correlation with thermal aging time. However, the fracture occurs on the Alloy 152 side and the fracture mode was the ductile fracture.

Effect of Ti/N Ratio on Grain Growth Inhibition in the Coarse-Grained Heat-Affected Zone of Low Carbon Steels Simulated for Ultra-High Heat Input and Tandem Electro-Gas Welding

Effect of Ti/N Ratio on Grain Growth Inhibition in the Coarse-Grained Heat-Affected Zone of Low Carbon Steels Simulated for Ultra-High Heat Input and Tandem Electro-Gas Welding

Effect of Internal Hydrogen on the Fatigue Crack Growth Rate in the Coarse-Grain Heat-Affected Zone of a CrMo Steel

The effect of internal hydrogen in the fatigue crack growth rate of the coarse grain region of a 2.25Cr1Mo steel welded joint was analyzed in this work. The microstructure of the coarse grain region was simulated by means of a heat treatment able to provide the same microstructure with a similar prior austenite grain size and hardness to the one in a real welded joint. The fatigue crack growth rate was measured under standard laboratory conditions using compact tensile (CT) specimens that were (i) uncharged and hydrogen pre-charged in a hydrogen pressure reactor (under 19.5 MPa and 450 °C for 21 h). The influence of fatigue frequency was assessed using frequencies of 10 Hz, 0.1 Hz, and 0.05 Hz. Additionally, two load ratios (R = 0.1 and R = 0.5) were applied to analyze their influence in the da/dN vs. ∆K curves and therefore in the fatigue crack growth rate. The embrittlement produced by the presence of internal hydrogen was clearly noticed at the beginning of the fatigue crack growth rate test (ΔK = 30 MPm), obtaining significant higher values than without hydrogen. This effect became more notorious as the test frequency decreased and the load ratio increased. At the same time, the failure mechanism changed from ductile (striations) to brittle (hydrogen decohesion) with intergranular fracture (IG) becoming the predominant failure mechanism under the highest loads (R = 0.5).

The Relationship Between Microstructure and Hardness in the Heat-Affected Zone of Line Pipe Steels

Welding is an essential fabrication step during the construction of pipelines using high-strength low-alloy steels. The base metal experiences a rapid thermal excursion in the heat-affected zone, resulting in modification of the microstructure and mechanical properties. In particular, the coarse-grain heat-affected zone closest to the weld pool is of significance for the integrity of pipelines. In the present work, the microstructure and hardness were quantified from laboratory-simulated samples, obtained at different cooling rates for 12 steels, where their chemistry was systematically varied. Electron backscattered diffraction was used to quantify the bainitic microstructures based on the density of high-angle grain boundaries (HAGBs). It was observed that an increase of the cooling rate results in (1) a reduction of the transformation start temperature, (2) a larger density of HAGBs, and (3) an increase in hardness. A Hall–Petch-type relationship is proposed to link the hardness with the HAGB density.

Microstructure and Impact Toughness of the Coarse-Grain HAZ Physically Simulated with Different Heat Inputs in a 690 MPa Ultra-low Carbon High-Strength Microalloyed Steel

Microstructure and Impact Toughness of the Coarse-Grain HAZ Physically Simulated with Different Heat Inputs in a 690 MPa Ultra-low Carbon High-Strength Microalloyed Steel

Physical simulation on Joining of 700 MC steel: A HAZ and CCT curve study

In the present work, coarse grain heat-affected zone (CGHAZ) was simulated by Gleeble 3800 thermo-mechanical simulator. A continuous cooling transformation (CCT) diagram was generated from the results of the dilatometer, hardness, and microstructure analysis. The heating rate of 100 °C s−1, the peak temperature of 1300 °C, the holding time of one second, and thirteen different cooling conditions representing the actual welding condition were chosen here for simulation where the cooling rate was controlled by t8/3. At slow cooling rate ferrite, cementite, pearlite, and bainite were obtained. At a medium cooling rate, ferrite, bainite, and a small amount of martensite were observed. At a fast-cooling rate i.e., 100 °C s−1 fully martensite was obtained. The obtained hardness values were 225 HV, 263 HV, and 342 HV for slower, medium, and fast cooling rates respectively. The increase in hardness value shows that the amount of non-diffusional phases increases with an increase in cooling rate. The CCT curve shows the range of cooling rate and phase transformation temperature of ferrite, bainite, and martensite.

Peculiarities of microstructure formation in the heat-affected zone of welded joints of pipes of x100 strength class

The features of the formation of the structure of coarse-grained heat-affected zone (CGHAZ) of a welded joint of pipe steel grade X100 are investigated depending on the cooling rate by simulating welding thermal cycles on thermo-mechanical Simulator – Gleeble 3800. As a result, a continuous cooling thermokinetic (CCT) diagram of austenite decomposition in the CGHAZ metal of pipe steel grade X100 was constructed, which makes it possible to study the structure and kinetics of phase transformations when the steel is exposed to the welding thermal cycle. It was found that the austenite decomposition is accompanied by bainitic and martensitic transformations in the range of cooling rates of 0.1–150 ºC/s. With an increase in the cooling rate, the bainite structure changes its morphology from a globular and coarse acicular to a dispersed acicular with a pronounced packet organization and a lath morphology, islands of the martensite-austenitic (MA)-constituents, which have an acute-angled shape and are distributed at the boundaries between laths, packets and inside them, are gradually transformed into thin layers. The martensite structure appears at a cooling rate of more than 15 ºC/s. Large blocks (crystals) of bainite with MA components can be observed in the structure despite an increase in the cooling rate. Hardness values change from 245–285 to 307–353 HV with the appearance and increase in the fraction of the martensite structure.

Effect of Nb on microstructure and mechanical properties between base metal and high heat input coarse-grain HAZ in a Ti-deoxidized low carbon high strength steel

In this paper, a Ti-deoxidized low carbon high strength steel plate with different Nb content was obtained by thermal mechanical control processing (TMCP) methods. The coarse-grain heat-affected zone (CGHAZ) under high heat input of 300 kJ/cm conditions was simulated. The effects of Nb on microstructure and mechanical properties in base metal (BM) and CGHAZ were analyzed. The results showed that high content of Nb addition is not conducive to the improvement of toughness, especially in high heat input CGHAZ. A certain amount of Nb inhibites the formation of ferrite, while promotes the bainite transformation with coarse carbides during high heat input welding process. Although high-angle grain boundary has a certain hindrance to the crack propagation, the coarse carbides caused by the enhanced hardenability in Nb-bearing steels stimulates the crack initiation sensitively. Besides, the uneven prior austenite grains may also take responsibility to the deterioration of toughness in Nb-bearing CGHAZ. In general, the content of Nb should be carefully added when impact toughness of CGHAZ is considered during high heat input welding in Ti-deoxidized low carbon high strength steel.

Effect of Welding Heat Input on Microstructure and Properties of Coarse-Grained HAZ of 500 MPa High-Strength Low-Alloy Steel

Effect of Welding Heat Input on Microstructure and Properties of Coarse-Grained HAZ of 500 MPa High-Strength Low-Alloy Steel

Effects of V–N Microalloying on Microstructure and Property in the Welding Heat Affected Zone of Constructional Steel

Shielded metal arc welding and welding thermal simulation experiment were carried out for constructional steel containing 0% V and 0.10% V, and the microstructure, precipitation feature, microhardness HV0.2, and −20 °C impact value in the welding heat affected zone (HAZ) were investigated. The results showed that in the coarse-grained heat affected zone (CGHAZ), V and N were completely dissolved in the matrix of steel containing 0.10% V to promote the growth of prior austenite grains, meanwhile the fraction of high angle grain boundaries (HAGBs) decreased, thereby leading to the mean −20 °C impact value decreases from 87 J to 18 J. In the grain refined heat affected zone (GRHAZ), V(C, N) precipitates experience re-dissolution and re-precipitation at grain boundaries, V–N microalloying changes the microstructure from lath bainite + granular bainite + small amount of polygonal ferrite to polygonal ferrite + pearlite + martensite, thereby leading to the mean microhardness decreases from 335 HV0.2 to 207 HV0.2, and the mean −20 °C impact value decreased from 117 J to 103 J. In the intercritical heat affected zone (ICHAZ), V(C, N) precipitates experience re-dissolution, re-precipitation, and growth, causing the formation of micro-sized V(C, N) precipitates, thereby leading to the mean −20 °C impact value decreases from 93 J to 62 J.

Evolution of Microstructure in Welding Heat-Affected Zone of G115 Steel with the Different Content of Boron.

Welding thermal simulation was performed to investigate the effects of boron content (0, 60, and 130 ppm), welding peak temperature (Tp), and cooling time from 800 to 500 °C (t8/5) on the microstructure, carbide, subgrain, and microhardness of heat-affected zone (HAZ) in G115 steel. According to the experimental results, the microstructure of coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), inter-critical HAZ (ICHAZ), and sub-critically HAZ (SCHAZ) was martensite, martensite containing a small amount of undissolved carbide, martensite, and over-tempered martensite, tempered martensite, respectively. The presence of B element improved the thermal stability of M23C6 carbide, thereby resulting in a greater amount of undissolved carbides with a larger diameter in the materials with higher B content under the same Tp. Element B is effective in improving Ac1 and Ac3 for the material. Besides, compared with the material without and containing 60 ppm B, the Ac1 and Ac3 of the material containing 130 ppm B increased by 95 and 108 °C, 69 and 77 °C, respectively. Meanwhile, the FGHAZ area of the material containing 130 ppm B was significantly lower than the material without or containing 60 ppm B, indicating that element B can significantly reduce the formation range of FGHAZ. The alloy content in austenite of ICHAZ of materials without or containing 60 ppm B increased, compared with CGHAZ, its Ms and Mf declined by 50 and 7 °C, 46 and 7 °C, respectively. In contrast, the alloy content in austenite of the material with 130 ppm B content decreases, its Ms and Mf was 37 °C and 32 °C higher than CGHAZ, respectively. The microhardness of HAZ was ranked in descending order as CGHAZ, FGHAZ, ICHAZ, and SCHAZ. Differently, the microhardness of CGHAZ and FGHAZ showed an increasing trend with the rise of B content but exhibited a decreasing trend with the rise of t8/5.

Effect of Ce on microstructures, carbides and mechanical properties in simulated coarse-grained heat-affected zone of 800-MPa high-strength low-alloy steel

The application of oxide metallurgy technology to induce the formation of acicular ferrite in coarse-grained heat-affected zone (CGHAZ) to improve the welding performance of steel is a research hotspot in recent years. However, the enhancement mechanism of mechanical properties after Ce addition and the behavior of microstructures in CGHAZ of 800-MPa high-strength low-alloy (HSLA) steel are rarely reported. In the present study, the effect of Ce addition on the welding performance of 800-MPa HSLA steel was investigated by analyzing the microstructures, carbides and mechanical properties in the simulated CGHAZ. The results indicated that after the welding thermal cycle, the decrease in toughness in the CGHAZ was mainly caused by the significant decrease in the crack propagation energy. Compared to the Ce-free steel, Ce addition improved the toughness of the base metal and CGHAZ by increasing the crack propagation energy. With the same heat input, the CGHAZ strength of the Ce-containing steel was higher than that of the Ce-free steel. During the welding thermal cycle, the Nb and Ti carbonitrides greatly affected the grain growth in the CGHAZ. Ce addition significantly refined these carbonitrides, providing a stronger pinning pressure to inhibit austenite grain growth in the CGHAZ. Ce addition also refined the austenite grains and increased the proportion of high-angle boundaries in the CGHAZ, increasing the impact absorb energy and decreasing the strength loss of the CGHAZ, essentially improving the welding performance of the HSLA steel. These results are helpful for clarify the mechanism by which Ce improved the welding performance of the 800-MPa HSLA steel.

Weldability and the Effect on Heat-Affected Zone Microstructure of a High Hardenability Boron Steel Compared to SAE 1045 After Shielded Metal Arc Welding

Abstract One of the steels commonly used in the industry in mechanical devices, tools, and structural steels is SAE 1045. A possible replacement for this alloy is SAE 15B30 with similar mechanical properties and has been used in the automotive and construction industry. To select an alloy, care is needed, for example, weldability. Making a comparative study of these alloys, can boron steel replace SAE 1045? Knowing that boron steels have high hardenability but may have improved weldability due to the low carbon equivalent. Therefore, it is extremely important to know the factors that can interfere with the quality of the final product after the welding processes, as the mechanical properties and microstructures in the heat-affected zone (HAZ) may change. Knowing that in the automotive and civil sector we have several welding processes, it is extremely important to know the results of a boron steel after a welding process; thus, this work intends to compare the weldability of SAE 15B30 and SAE 1045 welded by shielded metal arc welding (SMAW) and also to analyze the microstructural changes after welding by optical microscopy, scanning electron microscopy, and its relationship with hardness using the test of Vickers microhardness. To perform the welds, both in SAE 1045 steel and in SAE 15B30 steel, the butt joint with a V-shaped chamfer was chosen. The results showed that the boron steel, with high hardenability, obtained a microstructure with bainite in the coarse-grained heat-affected zone (CGHAZ), unlike SAE 1045, which presented pearlitic microstructure. In addition, the hardness of boron steel increased by approximately 90% over the base metal (BM), and the hardness of SAE 1045 increased close to 50% over the BM.

Effect of electrode negativity ratio and heat input on bead and arc characteristics in submerged arc welding

The effect of electrode negativity (EN) ratio, current and welding speed on bead profile and arc stability in case of constant current square wave power source were investigated by conducting bead on plate (BOP) welding experiments on API X80 steel. In addition to this, effects of heat input on prior austenite grain (PAG) size, microstructure and hardness of coarse grain heat affected zone (CGHAZ) were also studied. Deleterious hump angle created due to EN ratio variation, decreased with increase in EN ratio and welding current. Average PAG size and spacing between bainite laths increased from 17.09 µm to 62.66 µm and 0.67 µm to 50 µm respectively with increase in heat input from 0.69 kJ/mm to 3.6 kJ/mm, which altered CGHAZ hardness. Arc stability and metal transfer mode for different EN ratios and currents were analyzed by means of probability density distribution (PDD) graphs of current and voltage transients recorded using a digital storage oscilloscope. PDD graph show better arc stability at higher currents and during the electrode positive (EP) part of current waveform.

Mo Content Effect on Microstructures and Toughness of the Simulated Coarse-Grained Heat-Affected Zone of Weathering Bridge Steels

Mo Content Effect on Microstructures and Toughness of the Simulated Coarse-Grained Heat-Affected Zone of Weathering Bridge Steels

Resistance spot welding of high-strength DP steel and nano/ultrafine-grained IF steel sheets

In the current study, microstructural characteristics, mechanical behavior, and failure analysis of resistance spot welded joints for high-strength dual-phase (DP) steel and nano/ultrafine-grained interstitial free (IF) steel was investigated. The DP steel side indicated three different heat-affected zones including intercritical, fine-grained, and coarse-grained. The volume fraction of martensite in the coarse-grained heat-affected zone was higher than in other regions due to more growth of γ grains. The IF steel side exhibited three different subregions in the heat-affected zone, namely, ultrafine-grained, fine-grained, and coarse-grained heat-affected zone. In the coarse-grained heat-affected zone, the iron oxide was effective on the microstructure by suppressing the grain growth. The uniformity of microhardness in the fusion zone of 10 kA welding current was more than other samples due to faster homogenization of melt. All welded samples indicate hardening at fine-grained heat-affected zone (on both sides) and ultrafine-grained heat-affected zone (on the interstitial free steel side). With increasing the current of welding from 6 kA to 10 kA, the peak load increased from 7.38 kN to 11.23 kN and the energy absorption improved from 6.01 kJ to 11.72 kJ due to increasing the diameter of the fusion zone. All samples failed through partial thickness-partial pullout mode. Cleavage facets and shear dimples were observed in all samples, which indicated a mixture of brittle and ductile fractures.