Thick wear-resistant steel plates are utilized in challenging applications, which require a high hardness and toughness. However, utilization of the thick plates is problematic as they often have nonuniform mechanical properties along the thickness direction due to the manufacturing-induced segregations. In addition, the processing of thick plates commonly involves flame cutting, which causes several challenges. Flame cutting forms a heat-affected zone and generates high residual stresses during the cutting process. In the worst case, flame cutting causes cracking of the cut edge. The aim of this study is to investigate the role of plate thickness on the residual stress formation and cracking behavior when utilizing flame cutting. Residual stress profiles are measured by X-ray diffraction, plates and cut edges and are mechanically tested and characterized by electron microscopy. The results show that thicker plates generate more unfavorable residual stress state during flame cutting. Thick plates also contain segregations, which have decreased mechanical properties. The combination of high residual tensile stresses and segregations increase the risk of cracking during flame cutting. To prevent the cracking, the residual stresses should be lowered by lower cutting speeds and preheating. In addition, manufacturing practices should be aimed at lowering segregation formation in thick plates.
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In the worst-case scenario, flame cutting leads to cracking of the cut edge. The crack size can be several centimeters long. Commonly cracks are formed in the horizontal (sheet) plane, but they may also propagate in the vertical direction. In an earlier study[9] by the current author, it was found that cracks are often formed just beneath the flame cut surface and for that reason they are difficult to detect. The cracking probability has been noticed[12] to enhance with increasing plate hardness and thickness. In addition, Lindgren et al.[12] reported that high residual tensile stresses promote crack formation. It was shown that a residual compressive stress area is formed close to the cut edge, which is followed by a high residual tensile stress area deeper in the component. It was also noted that a slower cutting speed and flame cutting with preheating produce a lower residual stress state in the cut edge. Similar results were obtained in other studies[13,14] by the author; the residual stress formation during flame cutting can be affected by the size of the sample and cutting parameters. In addition, preheating not only decreases the residual stresses in general but also increases the compressive stress region close to the surface by shifting the tensile stress region deeper into the subsurface.
This study is a continuation for previous studies by the current author. The first study[14] confirmed that the residual state of the flame cut edge can be affected by variating the cutting parameters. In another study,[13] a model was developed to investigate the flame cut process and residual stress formation. It was used for example to investigate the thermal histories and residual stress formation of the modeled sample during flame cutting. The combination of preheating and low cutting speed produced the most beneficial residual stress state with the most compressive stress and the lowest tensile stress peak. To obtain further knowledge of the flame cut process and cracking behavior, another study[9] was made to investigate the microstructural characteristics related to this topic. It was shown that a higher residual stress state was caused by elongated prior austenite grains compared to an equiaxed prior austenite grain structure. In addition, the cracking probability is higher with an elongated prior austenite grain structure compared to equiaxed prior austenite grains. The elongated prior austenite grain structure is susceptible to cracking as it creates grain boundary chains, which can act as possible paths for crack formation. However, despite the studies mentioned earlier, there is still a considerable lack of knowledge on how the plate thickness affects the residual stress formation and cracking in flame cutting. The aim of this study is to determine the effect of the steel plate thickness on the residual stress formation and on the cracking probability. To achieve this goal, wear-resistant steel plates of varying thickness were mechanically tested, flame cut in controlled conditions, and inspected with ultrasound to evaluate the cracking tendency. After this the resulting residual stress state as well as the changes in the microstructure near the cut edge were characterized. The results of this study can be utilized to avoid the cracking of the plates during flame cutting and to improve the manufacturing of thick plate products.
Residual stress measurements were performed using an XStress 3000 X-ray diffractometer (manufactured by Stresstech Oy) by the modified Chi method.[16] The residual measuring method is described in more detail in previous studies.[9,13,14] The examination locations and directions of the residual stress measurements are described in Figure 1. As shown in the figure, measurements were performed at the centerline of the samples (where the cracks are mostly formed during flame cutting) in two perpendicular measurement directions: the rolling direction (0 deg) and the thickness direction (90 deg). The selected directions are the most important orientations from crack formation point of view.
The samples were inspected by ultrasound after flame cutting. As presented in Figure 12, no cracks were detected in the 20-mm plates after flame cutting. However, the inspection revealed three (1 crack/1600 mm) and four cracks (1 crack/975 mm) in the 40- and 60-mm plates, respectively. It should also be noted that no cracks were detected in the preheated samples. The cracks that were found were located immediately below the cut surface in the center region (i.e., near the midplane) of the plates. The crack normal was parallel to the thickness direction of the plate.
Ultrasound inspection of the flame cut samples revealed no cracks in the 20-mm plates. However, the cracking tendency is higher in thicker plates (40-mm plate: 1 crack/1600 mm and 60-mm plate: 1/975 mm). The inspection results indicate that thin plates withstand the residual stress state that is formed in flame cutting. In addition, most of the 40- and 60-mm plates were found to be intact after flame cutting. However, since some cracks were detected in the 40- and 60-mm plates, it can be stated that on some occasions the residual stress levels are high enough for crack formation. In addition, the impact toughness results are significantly lower in the thickness direction compared to other directions. It was also observed that the 40- and 60-mm plates contained more segregations in the center region than did the 20-mm plates. Thus, it seems that the cracks are commonly located in the segregations, which tend to be harder and more brittle than the base material. These results indicate that the susceptibility to cracking increases as the strong manufacturing-induced segregation occurring in the center region of thick plates is combined with the high residual tensile stress produced by high-speed flame cutting. However, all the tested steel plates exhibited ductile behavior macroscopically and most of the plates were found to be intact even in the presence of relatively large local residual tensile stresses. This highlights the stochastical nature of the cracking in flame cut steel plates. To lower the susceptibility to cracking, the residual tensile stresses should be lowered by utilizing the most optimal flame cutting parameters: low cutting speed and preheating. In addition, the formation of segregations should be avoided by improving manufacturing practices of thick plates.
Wear-resistant steel plates in three different thicknesses were investigated to study the effect of plate thickness on residual stress formation and cracking in flame cutting. The main conclusions of this study are summarized as follows:
Thicker plates tend to be more exposed to cracking. The susceptibility to cracking increases as the high residual stresses caused by flame cutting are combined with the manufacturing-induced, strong center segregation that occurs in thick plates. However, most plates were found to be intact even in the presence of relatively large local residual tensile stresses, and this highlights the stochastical nature of the cracking in flame cut steel plates.
The manufacturing of thick wear-resistant steel plates commonly leads to a layered structure and non-uniform properties in the thickness direction which makes the processing and utilization of the plates problematic. The processing steps of thick plates include flame cutting, which generates a heat-affected zone and high residual stresses into the cut edge. In the worst case, the cutting causes cracking. However, the residual stress level alone is not high enough to break a wear-resistant steel plate that behaves normally. Therefore, high-tensile stress also requires a microstructurally weak factor for crack initiation. For this reason, the main objective of this study is to reveal the main microstructural reasons behind the cracking of plates in flame cutting. To achieve this, plate samples containing cracks are mechanically tested and analyzed by electron microscopy. The results show that cracks are commonly formed horizontally into the tempered region of the heat-affected zone. Cracks initiate in the segregations, which typically have a higher amount of impurity and alloying elements. Increased impurity and alloying content in the segregations decreases the cohesion of the prior austenite grain boundaries. These weakened grain boundaries combined with high-residual tensile stress generate the cracks in the flame-cutting process.
In the worst case, flame cutting causes cracking of the cut edge. Cracks have been observed[7] to form close to the centerline of the plates. Generally, the cracks are a few millimeters in length but on rare occasions they can also propagate catastrophically through the whole plate. The smaller cracks are mostly formed just beneath the cut surface and are usually detected by ultrasonic inspection. Crack formation is exacerbated by residual tensile stresses while compressive stresses have the reverse effect. The microstructural features involved in residual stress formation and cracking behavior were evaluated by the current author in a previous study.[7] It was shown that an elongated prior austenite grain structure is more susceptible to cracking than an equiaxed prior austenite grain structure. The reason for this is that the elongated grain structure creates long, parallel prior austenite grain boundaries that can act as potential crack paths. In addition, the tendency for cracking increases as the plate thickness increases due to the higher number of segregations in thicker plates.[6] In addition, cracking of martensitic steels is often related to temper martensite embrittlement (TME) or temper embrittlement (TE).[12,13,14] TME occurs in tempering at around 300 C and is associated mainly with the formation of cementite at the interlath boundaries of the martensite or at the prior austenite grain boundaries. TE occurs after tempering or cooling through a temperature range of around 500 C and is associated mostly with segregation of impurities, such as antimony, phosphorus, tin, and arsenic into the prior austenite grain boundaries. Many studies[15,16,17,18,19] have concentrated on phosphorus, as it can be very detrimental for steel, even in trace amounts. However, the segregation of phosphorus and the cohesion of grain boundaries are also heavily affected by alloying elements.[20] The interaction of impurity and alloying elements has an effect on their segregation into the grain boundaries and eventually on the grain boundary cohesion. For example, nickel, manganese, and chromium strongly co-segregate with phosphorus and increase the risk of TE. In contrast, molybdenum also has a strong interaction with phosphorus and has a beneficial effect on TE by preventing phosphorus segregation and increasing the grain boundary cohesion.[12,20] In addition, hydrogen is typically very harmful for these kind of applications and the hydrogen embrittlement is a widely studied topic.[21,22,23,24] However, hydrogen embrittlement also requires susceptible microstructure and stress state which are the mainly focused in this study. 2ff7e9595c