Microstructure Of CGHAZ Research Articles

In Situ Observation of CGHAZ Microstructure Evolution of Hot‐Rolled Ship Plate Steel Containing 0.012 Yttrium under High‐Heat‐Input Welding

The laser scanning confocal microscope is used to simulate the microstructural evolution of a coarse‐grained zone composed of hot‐rolled ship steel with yttrium under high‐energy input welding. The results demonstrate that Y2O3 near the austenite grain boundary can effectively block the overall migration of the austenite grain boundary during the application of the welding heating process. Additionally, the presence of Y2O3 in the austenite grain can promote the formation of new grain and subgrain boundaries, thereby refining the austenite grain. During the postweld cooling process, the undercooling degree is high under a lower heat input, and the phase variation occurs along the direction at a large angle to the proto‐austenite boundary. Thus, a large amount of acicular ferrite (AF) is formed perpendicular to the proto‐austenite boundary. Based on the first principle, Y2O3 can be used as the nucleation site for AF phase transformation to promote the formation of a fine interlocking structure. However, its impact is limited because the cooling rate significantly affects the microstructure evolution of the coarse‐grain zone of the weld during welding with a high heat input.

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 heat input on microstructure and toughness of CGHAZ of deep-sea X70

Effect of heat input on microstructure and toughness of CGHAZ of deep-sea X70

A Comparative Analysis on Microstructure and Fracture Mechanism of X100 Pipeline Steel CGHAZ Between Laser Welding and Arc Welding

The objective of this study is to compare and contrast the differences in microstructure and fracture mechanism between CGHAZ of laser welding and submerged arc welding in X100 pipeline steel. The single-pass welding thermal simulation test of laser welding and submerged arc welding was carried out in MMS-200 thermal simulation test machine using self-made fixtures. The results indicated that the microstructure of laser welding CGHAZ and submerged arc welding CGHAZ was lath martensite and granular bainite, respectively. The impact energy of laser welding CGHAZ was 27 J, and the fracture mode was ductile fracture; the impact energy of submerged arc welding CGHAZ was 7.7 J, and the fracture mode was brittle fracture. The nucleation and propagation mechanism of cracks in different welding modes was established. Microcracks nucleated at the position of dislocation accumulation and remained austenite suppressed crack nucleation in laser welding CGHAZ. The nucleation site was at the M–A constituents in submerged arc welding CGHAZ. The main crack of laser welding CGHAZ was formed by one or a small number of microcracks. The main crack in submerged arc welding CGHAZ was formed by a large number of microcracks. The difference between crack nucleation and propagation mechanism is the reason why the impact energy of laser welding CGHAZ is higher than of submerged arc welding CGHAZ.

Effects of Zr addition on microstructure and toughness of simulated CGHAZ in high-strength low-alloy steels

The effect of Zr addition (0.005, 0.013, and 0.054 wt.%) on the microstructure and toughness of simulated coarse-grained heat-affected zone in high-strength low-alloy steels was revealed using a Gleeble 2000 thermal simulator. It was observed that elongated MnS inclusions were formed in the lowest Zr-containing steel, while only pure equiaxed ZrO2 existed in the 0.054Zr steel (Zr content of 0.054 wt.%). Complex oxide–sulfide inclusions (ZrO2 + MnS) with size of (1.40 ± 0.25) μm were formed in 0.013Zr steel (Zr content of 0.013 wt.%). The complex inclusions refined the prior austenite grain, and the nucleation of acicular ferrite was promoted compared to those of 0.005Zr steel (Zr content of 0.005 wt.%) and 0.054Zr steel. Consequently, the 0.013Zr steel possessed superior low-temperature impact toughness in relation to 0.005Zr and 0.054Zr steels. Thus, moderate Zr addition can be considered as an effective method to refine the structure and improve the mechanical properties of the coarse-grained heat-affected zone.

Small Fatigue Crack Initiation and Growth Behavior in Notch Root for Low Carbon Steel

To clarify the small fatigue crack initiation, growth and crack closure behavior at the stress concentration area of welding, microscopic successive observation with automatic microscope system was conducted for notch root of heat treated low carbon steel, which simulate coarse grain (CGHAZ) and fine grain (FGHAZ) heat affected zone (HAZ) microstructure. As a result, fatigue life of notched CGHAZ specimen was shorter than that of FGHAZ. In addition, to evaluate the small fatigue crack growth behavior at notch root, this study proposed to use crack opening displacement (COD) as a related value of the crack depth. The COD, which is related to small fatigue crack size, of CGHAZ grow faster than that of FGHAZ microstructure. Furthermore, small fatigue crack opening stress were directly measured with digital image correlation (DIC) analysis. Small fatigue crack closure at notch root were successfully measured with microscope system with DIC analysis. As a result, CGHAZ microstructure shows lower crack opening stress than FGHAZ. This is one of the reason why CGHAZ shows shorter fatigue life and faster COD growth rate than FGHAZ.

Microstructure and Properties of Weld CGHAZ under Different Heat Input for X90 Pipeline Steel

Two kinds of industry trial X90 pipeline steels which have different chemical composition were chosen as test objects, and the grain coarsening, microstructural characteristics and the variation rules of low-temperature impact toughness in weld CGHAZ of this two steel under different welding heat input were studied by physical thermal simulation technology, SEM, optical microscope and Charpy impact tests. The results showed that the microstructure in weld CGHAZ of 1# steel was mainly bainite ferrite (BF) and most of the M/A constituents were blocky or short rod-like; the grains of 2# steel were coarse and there was much granular bainite (GB). Meanwhile M/A constituents became coarser and their morphology changed from block to long bar; alloy content of X90 pipeline steel under different weld heat input had great effect on the grain size of original austenite. When heat input was lower than 20KJ/cm, the impact toughness in CGHAZ of lower alloy content pipeline steel was good; as heat input increased, impact toughness in CGHAZ of 1# steel increased to the values between 260J and 300J when heat input was between 20KJ/cm and 25KJ/cm and the dispersion of impact energy was small. The impact toughness of 2# steel decreased gradually and the impact energy had the obvious dispersion.

Microstructures and Properties of HAZ of JB800 Bainite Steel

JB800 steel has grain boundary allotriomorphic ferrite and granular bainite (FGBA/BG), and it is a kind of high strength low alloy steels, which has simple produce procedure, lower cost and excelled property. The law of microstructure transformation in CGHAZ, hardness, and impact toughness in HAZ of JB800 steel were studied by means of thermal simulation. The test results show that under the general condition of welding process (t8/5=5~50s), microstructure of CGHAZ is composed of mixture microstructure of Martensite and Bainite and with the increase of cooling rate, the content of Martensite will decrease, but that of Bainite will increase; when t8/5 is 20s, CGHAZ zone have better impact toughness, which is composed of 95% Martensite and 5% Bainite. Therefore t8/5 should be controlled at about 20s to get better impact toughness.

Prediction of CGHAZ Microstructure and Property of JG590 Steel with Weld Simulation Technology

A three-dimensional analytical calculation model of GMAW heat transfer was established, and the weld temperature field of JG590 with different welding parameters was analyzed by numerical simulation. The phase transformation in CGHAZ of JG590 steel was studied by physical simulation. SH-CCT diagrams, the effects of weld thermal cycle on microstructure, impact toughness and hardness were obtained. The results show that the critical cooling time of bainite, ferrite and pearlite appeared are respectively 8s, 110s and 150s. The critical cooling time that martensite takes to finish transferring is 35s. When t8/5 is below 10s, CGHAZ has a better toughness and when t8/5 is beyond 15s, there is a wide range decrease on the toughness. In this paper prediction of CGHAZ microstructure and property of JG590 steel was realized based on the analytical calculation model of GMAW heat transfer and physical simulated results mentioned above

Experimental Study on the Effect of CGHAZ Microstructure on CTOD of High-Strength Steel EQ56

The CGHAZ Microstructure has great effect on the toughness of welding joints and CTOD. This paper discussed the relationship between the CGHAZ and CTOD .The samples, carried out on high-strength steel EQ56, were observed by scanning electron microscopy. The Microstructural analysis result shows CGHAZ microstructure of EQ56-5, presenting high CTOD with 0.272mm, is low-carbon lath martensite and little M-A constituent, which contribute to the better toughness of EQ56-5. CGHAZ microstructure of EQ56-4, presenting the low CTOD with 0.082 mm, contains M-A constituent. The M-A constituent makes the CGHAZ brittle. The crack would grow easily along the M-A grain boundaries.

Effect of Niobium on HAZ Microstructure and Toughness of HSLA Steels

Experimental low alloy steels containing different levels of C, Nb and Mn were employed, to each of which a simulated thermal cycle was applied to produce CG-HAZ microstructure. Effect of Nb on the microstructure was observed and it was found that it has some beneficial effects, such as increasing the hardenability, which lead to retardation of GB ferrite and enhancement of intragranular ferrite formation in low and medium carbon steels. For high carbon steels, it enhances the carbides precipitation and as a result the MA area fraction decreases. The initiation site of brittle fracture was found to be dependent on the microstructure. Three categories of microstructure are described; namely grain boundary ferrite with Widmnastatten ferrite plus areas of degenerated pearlite, which is denoted as category I; grain boundary ferrite plus intragranular acicular ferrite which is called category 11; and finally the lath structure which is called category III. The brittle fracture was observed to be initiated at the pearlite colonies in category I, to be associated with the grain boundary ferrite in category II, and to be initiated at the intersection of the bainitic packets in category III. The area fraction ofMA increases by Nb addition for low and medium Carbon steels but for high Carbon steels, MA area fraction shows a slight increase due to carbides precipitation. MA is not likely to play an essential role in the brittle fracture initiation. Mn is the most dominant element affects the formation of MA at the same carbon equivalent. Therefore the beneficial effect ofNb could be obtained through the optimization of alloy design mainly Mn content.