Effect of torch process on the steels used for bucket, shovel handle, and other high-tonne mining equipment

In the air carbon arc cutting (CAC) process, the molten metal generated by the electric arc is swept off by a strong dry airflow rate of 700 to 1000 l/min and a working pressure up to 6 kg/cm². The electric arc is generated by a graphite electrode (typically 90% C) with other metallic elements (Cu, Ni, Fe, among others). The composition is chosen to facilitate the passage of current and to avoid corrosion of the sample due to the pressurized air (American Society of Heating 2011; Victor Technologies, Inc. 2013; Varkey et al. 2013; Gutiérrez 2006; IS 1979; U.S. Department of Transportation 1994; AWS 1980; Das and Abarna 2015). The most common uses of this technique are (a) preparations of welded joints, (b) removal of welding defects, and (c) removal of welding and joints of structures.

During the application of the CAC gouging torch process, it is possible to cause damage to the steel base substrate of the mining equipment, due to the high temperatures involved in the process, which can easily reach 2000 °C in the molten metal. At first, thermal fluctuations so as the addition of carbon concentration may cause effects on the thermally affected zone (HAZ) (Tingaev et al. 2016; Andrés et al. 2016), which may include residual stresses (Yilbas and Arif 2011; Michaleris 1999; Hu et al. 2007; Kyong-Ho and Lee 2007; Ma et al. 2016) and subsequently generate fatigue in the material (Goldberg 1978; Ramaswamy 1989; Cicero et al. 2017; Liu et al. 2016; Zhang et al. 2016; ASM 2013; Lara et al. 2013) or even spontaneous fracture, starting from micro-crack formation. It is also possible to suppose that solid-solid phase transformations so as micro-structure changes take place into the affected area during the cooling after the process end.

In maintenance work, a direct relationship between zones that have been treated through torching gauging processes and failure fractures due to fatigue or embrittlement has been found. For companies engaged in the minery industry, the recovery of abrasion-worn parts and the elimination of deteriorated areas in the mining equipment occupied in the extraction of high tonnage are extremely important.

The aim of this work is to study how the maintenance that uses CAC gouging torch process affects the microstructural behavior and mechanical properties in the most commonly used SAE 5130 and ASTM 514 S grade steels used in the manufacture of high-tonne mining equipment.

As were previously said, during the cut process, part of the material is melted by the effect of an electric arc. Since the chemical composition of the electrode is rich in carbon, it can happen that the bath modifies its composition increasing the original solute carbon content. If this happens, a change in the microstructure would be more likely to occur, not only because of the high temperatures involved but also because of the diffusion of C in the liquid steel in the bath where the cutting takes place. For this reason, it is interesting to know the composition of both the extracted material and the remnant in the piece, as part of the discussion related to the effects of the torch process.

As a first step, the composition of the material that comes out as slag from the cut is checked. This material is in principle liquid steel whose composition can be modified by the chemical elements that compose the electrode and that is solidified very quickly by the effect of the gas at high speed and pressure that extracts it from the bath generated once produced. The carbon content in these slag samples is higher than the original steel in both cases studied. For SAE 5130 steel, the C content determined in the slag was 1.69 wt% C, while for ASTM 514 grade S, a value of 1.93 wt% C was obtained. In Fig. 1, the microstructure obtained for ASTM 514 grade S ejected slag is shown in two different zones. Figure 1a corresponds to an austenitic structure, while Fig. 1b shows a microstructure of zone of high densities of needle carbides, consistent with a high-composition steel.

The microhardness is a reflect of the different structures. The obtained values fluctuate between 677 HV in the austenitic zone with dispersed carbides of Fig. 1a and 793 HV in the sample of Fig. 1b. The high dispersion in the structures could be derived from the different local cooling conditions, size of the slag drops, and other considerations during the application of the CAC procedure.

In the case of ASTM 514 grade S steel sample, rod-like and laminar carbides were found in the microstructure of the slag. Note that since the evaporation temperature of the graphite is 4827 °C, which is very close to the temperature that is reached at the electrode tip in the order of 5000–8000 °C, whereby the graphite of the electrode will evaporate, generating a reduction atmosphere near the cutting zone. While this gas is partially removed by pressurized air, carbon-rich areas could occur on the surface. In addition, the molten steel can facilitate the graphite entry since the temperature is higher enough to maintain the liquid bath for a short time. This allows the mixture of gas enriched with the steel to be produced by a diffusion mechanism through the gas + liquid interface, allowing the precipitation of carbides.

Figure 1b shows the typical dendritic microstructure of a foundry found in the cutting residue. The eutectic reaction L → ɣ + MC, together with the austenite, is evidenced in the observed structure. The formation of MC nuclei is originally in the form of sticks, later branching. Growth is in conjunction with austenite as the temperature decreases (Bochnowski et al. 2003, Yang et al. 2006, Hetzner and Geertruyden 2008, Luan et al. 2010).

The behavior of bulk material is of course of great interest. As a consequence of the high temperature of the CAC gouging torch process, a softening effect can be observed in the HAZ. The dimension of this zone is quite well defined and is a consequence of the thermal and geometric characteristics of the treated elements. This behavior can be observed by microhardness on a profile that begins on the outer surface of the sample and is directed towards the center of the same. The microhardness of unmodified SAE 5130 steel was 462 HV and for ASTM 514 grade S steel 260 HV. These values were taken in areas far enough away from the cutting zone and will be used as a reference for the analysis that follows. In principle, the surface hardness taken on the cutting zone does not show a substantive difference with the reference value. A little increasing was noted on ASTM 514 grade S, prior to decrease, and shows a softening starting from the 500 μm of depth. This effect of tempering has a depth that varied in each case, as can be seen in more detail in Fig. 2. The obtained values of hardness and microhardness in the profile are detailed in Tables 2 and 3.

Relative position (See text) (mm)	SAE 5130 (HV)
0.5	462
1	446
1.5	309
2	289
2.5	272
3.0	302
3.5	301
4.0	328
4.5	353
5.0	358
5.5	371
6.0	411
6.5	429
7.0	434
7.5	441
8.0	457
8.5	467
9.0	478
10	462
10.5	462

Relative position (see text) (mm)	ASTM 514 (HV)
0.5	285
1	246
1.5	216
2.0	236
2.5	257
3.0	245
3.5	242
4.0	243
4.5	253
5.0	249
5.5	251
6.0	250
7.0	260

As can be seen in Fig. 2, the HAZ area in the case of SAE 5130 can reach approximately 9 mm in depth. In this zone, the observed microstructure corresponds to martensitic type with a large number of Widmanstatten-type needles, as can be seen in Fig. 3.

The reduction of the microhardness could be a consequence of a tempering caused by a high temperature delivered by the heat energy delivered by the torch. As can be seen in the SEM images of Fig. 4 for SAE 5130 steel, in the zone close to the surface, fissures of the order of 20 μm are observed in the micrographs, although some may agglomerate to form larger cracks, several mm in length (1500 μm).

On the other side, the ASTM 514 grade S steel shows a small increase in surface hardness after the torch process, reaching a surface hardness of 285 HV. This value is an increase of approximately 10% with respect to the original value of 260 HV determined far away from the cutting zone. This increase was found within the 500 μm closest to the cut. As in the previous case, after this zone, a softening occurs in the interior of the sample, although the magnitude of the zone affected by heat is rather narrower than for the other steel, since in approximately 3 mm, it recovers practically the original mechanical behavior.

In the affected zone, the observed microstructure is martensitic, with ferrite Widmanstatten as shown in Fig. 5. Also, a large amount of porosity and microcracks of approximately 50 μm in length are found as can be seen in the scanning electron micrographs of Fig. 6.

The observed behavior was similar to the reported in the bibliography for other thermal cutting methods. For example, after application of oxyfuel, laser, and plasma cutting, in the samples of S460 M steel, an increase in the superficial hardness followed by a decreasing in the mechanical properties was reported (Andrés et al. 2016). Also, for S345 and S390 steels treated by oxyfuel and plasma, cutting was found with similar results after being subjected to thermal cutting (Tingaev et al. 2016).

The analysis of the hardness and the observed microstructure shows that the torch process influences the behavior of the parts to be repaired, affecting the mechanical properties of the area. Formation of microcracks and porosities was also observed due to the same process.

The carbon content of the substrate material shows a small increment, apparently affected by the roasting process. Also, the C content is increased in the residue, reaching values close to 2 wt% C.

In the bulk material, the SAE 5130 steel shows microcracks less than 20 μm in length and also some greater fissures up to 1480 μm. In the ASTM 514 grade S steel, formation of microcracks of up to 40 μm in length was observed, but not a greater scale fissuration.

In all cases, a surface hardness increase tendency was observed both for SAE 5130 as for ASTM 514 steels, up to a 500 μm deep. A deeper observation reveals a softening beyond this 500-μm zone, product of a tempering in the hot affected zone. The affected zone varies between studied steels. In the case of SAE 5130, the tempering reach 8000 μm deep, while for steel ASTM 514 grade S, the affected zone reaches 5500 μm.