Regular Article
Effect of Texture on the Cold Cracking in Weld Zone of T23 Steel
2015 Volume 55 Issue 1 Pages 308-311
Details
2015 Volume 55 Issue 1 Pages 308-311
Y-slit welded joints without preheating were used to prepare the cold cracks in T23 steel. Then the effect of texture on the cold cracking was studied from the views of macrotexture and microtexture. The results showed that the propagation path of the cold cracks was strongly affected by the macrotexture of the columnar grain structure after solidification. The fracture direction was changed at almost light-etched columnar grain but within each columnar grain an average fracture direction was maintained. The crack finally took the same initial direction before meeting another light-etched columnar grain. The strain distribution obtained by electron backscattered diffraction (EBSD) showed that different crystallography of the columnar grain induced large strain concentration, which led to the big angle transition of the main crack. In addition, transform texture, also named as microtexture in this study, had a large effect on the exact propagation paths of the cold cracks. The cold cracks propagated along lath boundaries which were almost parallel to {011}α plane.
In recent years, T23 bainitic materials has occurred to improve the efficiency and economy of fossil fuel generation.1) The T23 steel is well suited for boiler components working at lower temperatures, such as water panels for ultra super critical boilers, superheaters and reheaters for conventional boilers and heat recovery steam generators. In addition to its excellent workabililty, it has the advantage of being used without post-weld heat treatment (PWHT) when welding thin-walled tubes. Meanwhile, T23 is reported to be weldable without controlled preheat temperature.1) Therefore, owing to the good creep properties, Grade 23 is used sometimes prior to other grades with the advantage of lower cost.2)
Actually, it is clear from field and fabrication experience that T23 is potentially susceptible to hydrogen induced cracking (HIC) when welded without proper preheating.3,4) The cold crack will reduce the mechanical properties of the welded joint. This will shorten the life span of T23 steel and lead to destructive accident. Therefore, a controlled preheat temperature is required for T23 steel to avoid HIC.
The increased susceptibility to HIC for T23 steel may arise from the fact that these materials accumulate high levels of residual stress on-cooling from welding thermal cycles than their martensitic counterpart.1) Additionally, the hardness and microstructure in the root pass are critical factors of susceptibility to HIC during welding. In a word, hard microstructure in conjunction with high residual stresses and diffusible hydrogen due to improper shielding of the weld root can lead to HIC in the CGHAZ or weld metal.5)
Though the factors lead to the cold crack in T23 steel have been obtained, the exact propagation path can not be acquired by these factors. It is because the crack propagation depends strongly on the crystallographic orientation of the material relative to the crack tip. For example, cleavage fracture in lath martensite/bainite structures occurs on {110}α, {100}α, {112}α, {123}α6) which are also identified as secondary cleavage cracks propagation planes in the A 508 C1.3 bainitic steel.7) The hydrogen-related transgranular fracture observed in low carbon martensitic steels is characterized by cleavage fractures parallel to {011}M or {112}M glide planes.8,9,10) By using the etch pit method, the crystallographic orientation of the fracture morphology of hydrogen-assisted cracking is found to be the {110}α planes in HT 80 steel.4) It is obvious that, in bcc structure such as martenite and bainite, crystallographic orientation has an important effect on the propagation paths of the cold cracks.
Crystallographic analyses of bainite and martensite have been conventionally conducted by X-ray diffraction (XRD) which has the disadvantage of not providing geographical information of the microstructure.11) Another conventional method is transmission electron microscopy (TEM). Though it enables the correlation of accurate crystallographic information to the microstructure, the observed areas in TEM are often small (~10 μm) and analyses of the crystallography of larger areas are tedious and are therefore rarely carried out.11) Electron backscattered diffraction (EBSD) is a powerful tool for studying fracture mechanisms in steels on a microscopic scale.12) It can scan large area and simultaneously provide geographical information of the microstructure.
Researches on the cold crack in T23 steel mainly focused on the susceptibility and the factors lead to the cold crack.1,3,4,5) There are few related works about the effect of the crystallographic orientation on the propagation of the cold crack in T23 steel. In addition, only microtexture was considered in previous study.13) Actually, macrotexture induced by the columnar grains in the weld zone can also have an effect. The purpose of this paper is to assess the influence of the texture upon the cold crack propagation. The texture is studied at two different scales of the material. The cracking characteristics, as well as the strain concentration around the cracks, was analyzed to obtain the effect of macrotexture and microtexture on the cold cracking in the weld zone of T23 steel.
The cold cracks were produced by y-slit crack test without preheating. Figure 1 shows the size and dimensions of the specimen. The constraint and test welds were prepared by the shielded metal arc welding (SMAW) with CHROMET 23L electrodes of Φ3.2 mm preheated for 1 h at 350°C. The nominal compositions of T23 base metal and electrode used in this study are given in Table 1. The welding variables utilized in this study are listed in Table 2.
Schematic diagram of specimen for y-slit cracking test.
| C | Mn | P | S | Si | Cr | Mo | W | V | Nb | N | Al | B | Ni | Ti | Ti/N | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| BM | 0.06 | 0.37 | 0.012 | 0.002 | 0.28 | 2.32 | 0.08 | 1.56 | 0.22 | 0.044 | 0.0035 | < 0.015 | 0.0026 | 0.05 | 0.009 | 3.9 |
| Electrode | 0.05 | 0.5 | 0.01 | 0.01 | 0.25 | 2.2 | 0.2 | 1.6 | 0.23 | 0.03 | 0.02 | – | 0.001 | 0.6 | – | – |
| welding current | arc voltage | welding speed |
|---|---|---|
| 110 A | 22–24 V | 1.7 mm/s |
Cross sections of the test weld were taken, polished and etched with a 4% nital in order to observe the propagation feature of the cracks by optical microscopy (OM). Then the sections were polished again and followed by a vibratory polish with 0.05 μm colloidal silica solution to finish it to a smooth and flat surface. A NOVA NanoSEM 230 microscope equipped with an AZtec system was used in this work. EBSD analysis was carried out in the vicinity of the main crack and microcracks. EBSD scans were performed with an accelerating voltage of 20 kV, working distance 10 mm, spot size 3. A scan step size of 0.2 μm was used at 3000x magnification. Acquired data were evaluated by Channel 5 software. Fractographic observations of the fractured specimens were also made on a NOVA NanoSEM 230 microscope.
A low magnification micrograph of the sample is shown in Fig. 2(a). The crack initiated due to the stress concentration at the sharp notch. Then the crack first propagated along the fusion line and then into the weld zone in a wavy way. The way of crack propagation was common in HIC.14,15,16,17,18) The arrow 1 shows that the crack is deviated between two columnar grains with different etching degrees. The crack finally took the same initial direction almost paralleling to the fusion line just before meeting another light-etched columnar grain outlined by arrow 2. The fracture direction was changed at almost light-etched columnar grains but within each columnar grain an average fracture direction was maintained.
Optical macroscope image of the crack (a) and SEM images of fracture surfaces in (b) weld zone and (c) heat affected zone.
Two types of fracture surfaces are observed in the specimen, as shown in Figs. 2(b) and 2(c). It showed that the fracture surface in the heat affected zone was characterized by fine dimples. The appearance of the fracture in the weld zone suggested that it was of a brittle nature. The fracture surface had facets parallel to a specific set of crystallographic planes.
During the solidification of steel welds, δ-ferrite grains grew epitaxially from the fusion surface into the weld pool, forming the classic columnar grain structure. The major axes of the columnar grains tended to lie along the direction of maximum heat flow. The <100> axes of δ-ferrite paralleled mostly to the heat flow direction. Then, the grains that were badly orientated in this respect were stifled at an early stage of solidification, by the rapidly growing grains.19) Such as the dark-etched columnar grains that are vertical to the fusion line in Fig. 2(a). It could be found that the deviation of the crack propagation direction was induced by different orientation of the columnar grains. The crystallographic anisotropy played a significant role in the mechanical properties of the material, which strongly affected the propagation of the crack.
Figure 3 shows one of the locations where the main crack is deviated due to the macrotexture of the columnar grains. Strain distribution around the main crack is displayed by kernel average misorientation (KAM) maps in which different colors stand for various degrees of strain concentration. In order to highlight the strain concentration at this deviation, the KAM values less than 2 have been removed. Figure 3(a) exhibits the scanning zones with two square grid regions corresponding to Figs. 3(b) and 3(c). The propagation direction of the crack parallels to the columnar grains in Fig. 3(b) and is vertical to the columnar grains in Fig. 3(c). Combining Figs. 3(b) and 3(c), it was clear that the deviation of the main crack was induced by the obvious large strain concentration circled with red dotted lines. Different crystallographic orientation of the columnar grains, along with the anisotropy of the large non-equixed columnar grain itself, resulted in the differences in the mechanical properties. Nonuniform distribution of the mechanical properties caused large strain concentration, which affected the propagation path of the cold cracks. This further illustrated the cold crack propagation had a relationship with the local stress level rather than the average stress level of the welded joint.
(a) Schematic diagram showing the scanning zones of the deviation, (b) and (c) the strain distribution corresponding to the two regions in (a).
In order to analyzing the microtexture of the cold crack, orientation mapping by EBSD on the sections which were vertical to welding direction has been carried out. Figure 4(a) shows the inverse pole figure (IPF) map of the microcracks. The correspondence between color and crystal orientation was shown in the stereographic triangle inserted at left upper corner. The laths circled with line belong to the right packet are separated by the microcrack, forming unflatness cracking faces, as shown in Fig. 4(b). It showed that crack propagated along lath/block boundaries rather than packet boundaries. The crack was deviated at almost each lath/block boundary but an average crack direction was maintained. The result was similar to reference 6. The cracks were arrested at packet boundary, because the high angle boundaries could efficiently arrest the propagation of cleavage cracks.20)
IPF maps of microcracks on (a) packet boundary and (b) enlarged view of the dotted square grid in (a), (c) <001>bcc pole figure corresponding to the solid square grid in (a).
Figure 4(c) is {001} pole figure which shows the orientations of the lath within the prior austenite that corresponds to the area surround by the black solid square grid in the IPF map. According to references,7,21,22) the concentrations near the ideal orientations based on the K-S orientation relationship are obvious, which confirms that the martensite and lower bainite in the T23 steel under investigation maintains the K-S orientation relationship. Though the orientation distribution is not discrete, it has a certain degree of scattering around the ideal orientations. Other scans can obtain the same orientation relationship. Previous studies reported that in the martenite with KS orientation relationship, the boundaries between adjacent blocks or adjacent laths inside a packet have the identical crystallographic plane that corresponds approximately to a {011}α plane.21,23) In this study, the crack just propagates along the lath/block boundaries. In addition, there is a crystallographic similarity between lath martensite and lower bainite in a same steel.24) Therefore, the cracking plane is {011}α plane. This conforms to crystallographic features of hydrogen induced cracking. It is because the predominant trapping sites of hydrogen in lath martensite are interfaces between laths or prior austenite grain boundaries.25,26) Moreover, Kim et al.27,28) discussed that the hydrogen induced cracks propagated along martensite lath boundaries because the lath boundaries are almost parallel to {011}α. In conclusion, transform texture has a great effect on the crack propagation path, which can be verified through the strain distribution near the cracks.
Figure 5 shows a large strain concentration at the initiation location of the microcrack, which leads to a microcrack. Compared with Figs. 3(b) and 3(c), the strain concentration is obvious smaller. This states that transform texture directly affect the crack propagation when macrotexture stays the same. In conclusion, the strain concentration along the lath boundaries which are almost parallel to {011}α, lead to the crack propagating along the lath boundaries.
Strain contouring map of KAM overlaid on the band contrast (BC) map around microcracks.
When the distribution of the global residual tensile stress stays the same in the weld zone, the propagation paths of crack depend on the macrotexture and microtexture of the material. The big angle transition of the main crack is largely influenced by the macrotexture led by the solidification through changing the distribution of local strain. Besides the effect of the microtexture on the local strain, the exact propagation paths are controlled by predominant hydrogen trapping sites of the microtexture. The cold cracks propagated along lath boundaries which are almost parallel to {011}α plane. The strain concentration along the lath boundaries which are almost parallel to {011}α. It verified that the crystallographic anisotropy played a significant role in the mechanical properties of the material.
The present work is supported by the National Natural Science Foundation of China (No.50975176), 04 Project (2012ZX04010-091), and Science and Technology Commission of Shanghai Municipality (10dz1201200).
