COMPUTER MICRO-IMAGE ANALYSIS IN THE Li Yajiang" and Wang Juan* ABSTRACT The microstructure and grain degree in the heat-affected zone (HAZ) of HQ130 high strength steel, which was welded by gas shielded metal arc welding, are researched by means of microscope and computer technique. A series of sample are cut from the heat affected zone of HQ130 steel to make metallographic analysis. The relative content of different microstructure in the HAZ is researched by means of XQF-2000 micro-image analyzer. Fine structures in the HAZ are analyzed by means of Transmission Electron Microscope (TEM) and electron diffraction. Experimental results indicate that with the increase of weld heat input (E), cooling time (t8/s) is longer and impact toughness deteriorates. The main microstructure in the HAZ changes from lath martensite (ML) and low bainite (ML+BL) to lath martensite, low bainite and upper bainite (ML+BL+Bu), and the grain becomes coarser, especially, the austenitic grain diameter increases to 158μm when weld heat input (E) is 22.3kJ/cm. By controlling weld heat input under 16kJ/cm to form ML+BL in the HAZ, avoiding producing Bu and preventing grain coarse, the microstructure and performance in the HAZ of HQ130 steel can be ensured. KEY WORDS High strength steel, Heat-affected zone, Microstructure and Micro-image INTRODUCTION HQ130 steel is newly developed low alloy high strength steel used in welded structures, the tensile strength of which is 1300MPa. The various parts in the heat affected zone (HAZ) of HQ130 steel experience different weld thermal cycle. The HAZ is considered as a weak part in the welded joint of the high strength steel (Ref.1,2). It is very important to ensure the microstructure and mechanical properties of welding zone, especially for the low alloy high strength steel. Previously researchers made some qualitative analysis in the heat-affected zone of high strength steel by means of traditional metallography (Ref.3-5). *School of Materials Science and Engineering, Shandong University, Jinan 250061, P.R.China E-mail: yajli@jn-public.sd.cninfo.net Operators observe the microstructure under the metallograph with slow velocity and high requirement for the operator's skill and the result with inaccuracy. The use of advanced computer technology to analyze the microstructure in the weld zone, will be significantly increase the accuracy of determining the microstructure in the HAZ. In this paper, the toughness in the HAZ of HQ130 has been studied through the weld heat simulated test. We have measured the relative content of different microstructure and grain degree in the HAZ by means of XQF-2000 micro-image analyzer, which will decrease the artificial influence to analyze the microstructure. We also make further analysis of fine microstructure in the HAZ by means of Transmission Electron Microscope (TEM) and Scanning Electron Microscope (SEM), which provide experimental and theoretical basis for the determining welding technology parameters. EXPERIMENTAL METHODS Chemical compositions and mechanical properties of HQ130 steel after the heat treatment are listed in Table 1. The thickness of the base metal is 12 mm in the test. Using gas shielded metal arc welding (GSMAW) with single V groove at the place of butt joint and with different weld heat input (E), such as smaller (9.6kJ/cm), middle (16.0 kJ/cm) and larger (22.3kJ/cm). Chemical compositions and mechanical properties of HQ130 high strength steel Table 1 The weld thermo-simulation test of HQ130 steel was carried out by means of a Gleeble-1500 test machine. The test parameters were: peak temperature (Tp) 1350°C and cooling time (18/5 ) 5s, 10s, 20s, 40s. A series of sample are cut from the heat-affected zone of HQ130 to make metallographic analysis. The relative content of microstructure in the HAZ is measured by means of XQF-2000 micro-image analyzer. The relative content of different microstructure in the HAZ is calculated using the computer according to the percent that the microstructure represents by each color covering the field of vision. The grain degree was measured by the cutting-line method. The formula of grain degree index is: G=-3.2877+6.6439 lg (mXN/L) Where: L is the gauged lattice length (mm); M is the magnifying multiple; N is the cutting dot number on the gauged lattice. Thin slice from the HAZ by line-cutting machine was made into film sample. The fine microstructure was analyzed using TEM and electron diffraction in the chosen zone. EXPERIMENTAL RESULTS AND ANALYSIS Impact toughness in the HAZ Impact work and hardness in the simulated HAZ have close relationship between peak temperature (7p) and cooling time (18/5). When Tp-1350°C, the result of thermo-simulation test for impact work and hardness in the HAZ of HQ130 steel is listed in Table 2. E/ kJ.cm Impact toughness and hardness in the simulated HAZ of HQ130 steel (7p=1350°C) Akv /J HV (53.7) (54.0) (26.6) (62.4) (255) Note: 1) Relationship between t8/5 and E was calculated by using the D.Uwer formula (Ref.3); 2) Number in the bracket is the test average value. When Tp=1350°C,which is corresponded to the coarse grain heat-affected zone(CGHAZ) and 18/5-40s (with considerable weld heat input), impact work decreases obviously. The test results indicate that t8/5 should be controlled smaller than 20s, because the larger the weld heat input is, the longer t8/5 and the easier the deterioration of impact toughness in the HAZ is. In practice, most welded joints are formed by the multi-layer welding technology. The heat-affected zone will undergo multiple weld thermal cycles. Impact work in the HAZ of multi-layer welding is obviously increased, which is favorable to welding of HQ130 high strength steel. The test result of impact work in the practical welded joint zone of HQ130 steel is shown in Table 3. Table 3 Positions Impact work in the practical HAZ of HQ130 high strength steel Coarse grain zone The fusion zone 97.1, 99.6, 111.8 Base metal 60.5, 63.2, 68.8 (64.2) Note: Impact work in the fusion zone virtually included a part of weld metal and HAZ. Microstructure in the HAZ of HQ130 The austenite grain in the CGHAZ near the fusion zone for HQ130 steel grows up obviously and grows into even-distributed austenite, which will change into coarse low carbon lath martensite (ML) when cooling rapidly after the welding (see Fig. 1a). The coherent lattice interface lies between the ML and the former austenite, which is non-diffusion phase transform. Fig.1b shows the feature of the lath martensite (ML) in the CGHAZ and the fusion zone of HQ130 observed under SEM. The main feature is that the low-carbon martensite lath is distributed in bigger angle and Matensite laths are nearly parallel to form a martensite zone. If the former austenite grain is finer the width of the martensite lath would be smaller and toughness would be better. Figure 1 a) CGHAZ 1500X b) CGHAZ and the fusion zone 1500X Feature of microstructure in the HAZ of HQ130 high strength steel Metallographic analysis shows that there is low banite (BL) in the coarse grain heat affected zone (CGHAZ) of HQ130 steel besides a large number of ML. Some of BL are parallel to ML, others transverse martensite lath and cut apart the austenite grain. When austenite changes into ML during the cooling, more than ten closed lathes have the same orientation and make up a bond of lathes. When BL+ML mixture microstructure is produced in the CGHAZ, the first separated BL would carve the former austenite effectively, which makes more opportunities for ML core and limits the growth of ML. In contrast to a unitary ML, the mixed microstructure of BL+ML has more larger angle boundary. Crack developing is hindered on the border of ML bunch or ML-BL border and changes direction, the length of crack developing in unit becomes shorter, so the impact toughness increases obviously. The relative content of microstructure and grain degree The relative content of various microstructures is not easy to be determined accurately. The contents of the BL and Bu are very important to toughness in the HAZ. The type of microstructure and their relative contents in the HAZ with different weld heat input were analyzed by means of XQF-2000 micro-image analysator. The result of quantitative analysis is listed in Table 4. Effect of weld heat input on microstructure is showed in Fig.2. The larger weld heat input is, the more easily the Bu would be produced in the HAZ. When the weld heat input (E) is controlled under 16.0 kJ/cm, the mixed microstructure of ML+BL may be acquired in the CGHAZ where there is no Bu. A certain angle exists among BL lath and separates martensite lath, which leads to the increase of sub-boundary and the decrease of crack sensitivity and make the toughness in the HAZ increase obviously. When the weld heat input is 22.3 kJ/cm, the main microstructure in the HAZ is ML+BL+Bu, among which there is 9.8 percent Bu. The orientation of crystallization among Bu lath is very small, cleavage crack can run through Bu lath continuously, corresponding to lower cleavage fracture stress, which makes the toughness decrease in the HAZ. The weld heat input in welding of HQ130 steel should be limited (E≤16 kJ/cm) in order not to produce Bu in the HAZ. Table 4 Relative content of microstructure in the HAZ of HQ130 steel Weld heat input /kJ.cm1 Note: Number in the bracket is the average value of three version field Figure 2 Effect of weld heat input (E) on the relative content of microstructure in the HAZ Affected by the peak temperature, the austenite grains become coarse in the CGHAZ of HQ130 steel. The width of the CGHAZ and the diameter of austenite grain are different using different weld heat input. Under the condition of different weld heat input, the width of the CGHAZ and austenite grain degree is evaluated using XQF-2000 micro-image analyzer, and the grain diameters are calculated by means of the computer. The analysis results are shown in Table 5. the width, the grain degree and diameter in the CGHAZ of HQ130 steel Table 5 Width of CGHAZ Maximum grain 9.6 /mm 1.03, 1.10, 0.98 Grain degree diameter /um 105 (3.6) Note: Number in the bracket is the average value of three version field |