Microstructure of HAZ of QT and TMCP HSS

Microstructure of the metal surrounding weld interface is influenced by heat while the weld joint is being formed. In the studied welded joint of QT HSS E and TMCP HSS C CGHAZ, FGHAZ, ICHAZ and SCHAZ are clearly recognized.

The microstructure changes continuously depending upon the maximum tem-perature attained in each region of the HAZ.

Close to the weld interface the metal is exposed to the temperatures between liquidus and solidus lines described as the fusion line (FL). This zone is in par-tially melted state. Microstructure of FL of QT HSS E has mixed microstructure which contains bainite and polygonal ferrite, fig. 63.

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Figure 63. Optical microstructure of the fusion line of QT HSS E.

QT HSS E CGHAZ borders the FL and refers to the HAZ subjected to peak temperatures above the grain coarsening temperature, the latter is 1300 ^oC for steels which have been Ti-treated to elevate their grain coarsening temperature (Eastling 1992). As the peak temperature exceeds the critical point, AC3, complete retransformation to austenite occurs, fig. 64. The extent of following grain coarsening depends on the peak temperature, the time above the grain coarsening temperature, the chemical composition of the steel and presence of undissolved nitride and carbonitride particles. When heated above 1300 ^oC, most of these particles, except the most stable such as TiN, dissolve (Mitchell et al. 1995). This results in reduction of pinning effect of the particles and following grain growth. At the same time long exposure of the HAZ to high temperature promotes homogenizing of austenite by alloying elements. So grain coarsening and homogenizing of austenite make it more stable. During cooling the grain coarsened austenite transforms to non-equilibrium transformation products depending on steel chemistry and cooling rate.

140 Figure 64. Scheme of CGHAZ formation.

In both TMCP and QT HSS steel coarse grain microstructure of initial austenite grains is clearly revealed in CGHAZ. Austenite grains grew from 5.6 µm, num-ber 12 (base metal) up to 75 µm, numnum-ber 4-5 (according to ASTM E112-10) during welding heating. In QT HSS E during subsequent cooling coarse grains were divided into packets of a lath bainite and low-carbon martensite, which slightly refines the constituents of the structure and has a positive effect on the resistance to crack propagation (Lamberte-Perlade et al. 2004). In TMCP HSS C during subsequent cooling coarse grains were divided into packets of a lath and granular bainite. Both microstructures are seeing in fig. 65 a and b.

a) b)

Figure 65. Optical microstructure of CGHAZ of QT HSS E (a) and TMCP HSS C (b).

Identification of structural constituents was derived from measuring their microhardness. Microhardness indentation was conducted by Vickers scale and

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0.025 kgf loading. The hardness of the lath martensite in CGHAZ exceeds 300 HV and reaches 340 HV, fig. 66. Packets of bainite have hardness less than 300 HV. Tempered martensite of the base metal is characterized by a hardness of 270-280 HV.

In the present investigation, martensite and bainite are distinguished by quite different etching susceptibilities as shown by optical micrographs, fig. 66. Since the bainitic transformation occurs at a higher temperature compared to the martensitic transformation, carbon can diffuse to a greater extent either to the remaining austenite islands or to the boundary between laths (Thewlis 2004).

When this structure is etched, the boundaries of the retained austenite islands or its decomposition products etch deeply, giving the overall appearance of a plate shaped ferritic matrix with a superimposed dispersion of dark contrasting particles. The martensitic transformation is characterized by clusters of very fine ferritic laths which form at lower temperatures. Since the carbon distribution in the martensitic structure is more uniform, it etches more evenly.

Figure 66. Microhardness measurement in CGHAZ of QT HSS Е.

The microstructure of the CGHAZ of TMCP HSS C formed during weld thermal cycle consists of the products of bainite transformation of austenite, fig. 67.

These microstructures are classified as bainite which may take many morphologies. Bainite-ferrite is one example of a microstructure which consists of a carbide-free ferrite matrix with well-defined islands of retained austenite or martinsite-austenite (M-A) constituent. The microstructure of granular ferrite

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consists of dispersed retained austenite or M-A constituent in a featureless matrix which may retain the prior austenite grain boundary structure (Krauss G

& Thompson 1995).

Most prior austenite grain boundaries are clearly visible in CGHAZ of TMCP HSS C, allowing the mean austenite grain size to be measured. The mean austenite grain size at this size is 89.0 µm, 4 number (according to ASTM E112-10). Within prior austenite grain several crystallographic packets with high misorientation angles between them, which slightly refines effective grain size, can be identified.

Figure 67. Optical microstructure of CGHAZ of TMCP HSS C.

As determined in CTOD and Charpy-V tests, a coarse microstructure decreases impact ductility. Charpy-V values of CGHAZ TMCP HSSs were good but some QT HSS steels had low impact ductility values. CTOD test values of Gleeble made CGHAZ test bars were very low. Impact ductility of bainite microstructure is higher than martensite microstructure.

FGHAZ refers to HAZ regions which have been subjected to peak temperatures between the austenite grain coarsening temperature and the upper critical point AC3, typically between about 1300 and 910 °C (Eastling 1992). Both CGHAZ and FGHAZ are the zones which have become fully austenitic due to weld thermal cycle. The microstructures of these zones continuously change

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according to the former austenite grain size. Consequently, it is difficult to precisely indicate the boundary between CGHAZ and FGHAZ.

The reduction in peak temperatures in this zone implies that, following the α→γ transformation during heating, the austenite does not have time to develop properly, and the grain size remains small. In addition, nitrides and carbides may not be fully dissolved, fig. 68.

During α→γ transformation γ grains nucleate heterogeneously at the bounda-ries prior γ grain and grow along them. Also the nucleation of γ grains occurs due to the dissolution of cementite, fig. 68. During γ→α transformation, the large grain boundary area tends to promote nucleation of fine ferrite grains.

Figure 68. Scheme of FGHAZ formation.

Along the HAZ of HSS QT steel, FGHAZ has the most fine grain structure with the mean grain size of 4.0 µm, 13 number (according to ASTM E112-10), fig.

69. There are more equilibrium transformation products, such as polygonal fer-rite, and islands of granular bainite in this zone. Compared with tempered mar-tensite of BM, microstructure constituents of FGHAZ have lower hardness.

Hardness of ferrite equals 210 HV, granular bainite 230 HV, fig. 70.

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Figure 69. Optical microstructure of FGHAZ of QT HSS E.

Figure 70. Microhardness measurement in FGHAZ of QT HSS E.

As a result of rapid heating and short exposure to high temperatures, the homogenization of austenite is not completed and some islands of retained austenite are enriched by carbon, that could promote formation of martensite or transformation to perlite in these islands.

The most fine grain and uniform structure within the HAZ of TMCP HSS C is observed in FGHAZ, fig. 71. The microstructure contains mostly polygonal fer-rite with a hardness of 220 HV and dispersed islands of granular bainite with a hardness of 240 HV.

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Figure 71. Optical microstructure of FGHAZ of TMCP HSS C.

ICHAZ refers to HAZ regions which have been subjected to peak temperatures between the upper and lower critical points AC3 and AC1, typically between 910 and 720 ^°C (Eastling 1992). In this region partial retransformation to austenite occurs during heating, the exact extent of which is governed by the peak temperature within the intercritical temperature range. During cooling, the austenite regions decompose to different extents and to various transformation products, fig. 72 (Matsuda et al. 1996).

Figure 72. Scheme of ICHAZ formation.

The microstructure in this region consists of a mixture of bainite, tempered martensite and perlite, fig. 73. Carbides, mainly cementite also experience a process of spheroidization and coagulation.

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Figure 73. Optical microstructure of ICHAZ of QT HSS E.

The SCHAZ is the region of HAZ that has been subjected to peak temperatures below the lower critical point AC1, below 720 ^°C (Eastling 1992). The processes of nucleation and spheroidization of carbides occurs in this zone, fig. 74. Black cementite conglomerates are clearly identified in fig. 75. The agglomeration of spheroidized cementite particles at grain boundaries and triple junctions em-phasizes the role of grain boundaries as high diffusivity channels for carbon at these low temperatures.

Figure 74. Scheme of SCHAZ formation.

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Figure 75. Optical microstructure of SCHAZ of QT HSS E.

The ICHAZ and SCHAZ regions of TMCP HSS, fig. 76 a and b, can be hardly distinguished unlike the HAZ of steel QT, fig. 73 and 75. This happens because the TMCP steel has a low carbon content and heating up to temperatures around critical point AC1 does not produce large scale nucleation of cementite and its coagulation.

a) b)

Figure 76. Optical microstructure of TMCP HSS C: a) ICHAZ, b) SCHAZ.