Influence of Welding Regimes and Filler Metals on Hardfaced Layers for Precision Mechanical Components Made of 30CrMoV9 Steel

Low-alloy steels are widely used in precision mechanical components because they provide a favorable combination of strength, hardness, and toughness at a reasonable cost, which makes them suitable for demanding parts such as gears, shafts, and tooling [1], [2]. During service, components made of these steels are often subjected to wear and degradation, which may reduce their load-bearing capacity and functional reliability, making repair or replacement necessary. Hardfacing is a widely used repair technique in which a layer of hard, wear-resistant material is deposited onto a worn surface in order to restore its functionality. Welding is one of the most commonly used methods for applying hardfacing layers, using appropriate filler metals as the deposited material. It is particularly attractive for repair applications because it can be applied to components with complex geometries using different joint configurations and welding techniques [3], [4], [5]. Therefore, the selection of suitable filler metals and welding parameters is a key issue in ensuring the reliability of repaired components. It is also essential to understand how repair welding affects the mechanical properties of the weld bead and whether the load-bearing capacity of the repaired component can approach that of the original material. In many cases, post-weld heat treatment is required to achieve mechanical properties comparable to those of the base material (BM). Accordingly, this paper investigates hardfacing by welding on plate specimens made of low-alloy steel 30CrMoV9, using different filler metals and welding regimes [6], [7], [8], [9].

The steel plates with dimensions of 150 × 80 × 10 mm were used for hardface welding. The BM is the low-alloy steel 30CrMoV9 of chemical composition given in Table 1 [1].

Weld beads of various fillers were applied onto the 30CrMoV9 plates (Figure 1) by different single pass welding regimes of semi-automatic welding, using $\mathrm{CO}_2$ protective gas (C type [2]). Four fillers in the form gas shielding welding wire (diameter 1.2 mm) and one filler as flux-cored wire (No. 5) were used. The chemical composition of the fillers is given in Table 2.

The welding input parameters, electricity, voltage, and welding speed (travel speed of the welding torch) are varied for every type of filler metal. Each of the five used fillers was welded on the plate by two regimes. The welding regime parameters are presented in Table 3.

Based on the values of current $I$, voltage $U$, and welding speed $v$, the energy supplied by the welding arc to the hardfacing plate is calculated by the following equation:

Calculated arc energy, depending on welding fillers and regimes is given in Table 3. Heat input considers the influence which process efficiency has on the energy that reaches the workpiece to form the weld. $HI$ is given by the equation:

Heat input, calculated based on arc energy considering the efficiency of 75% is given in Table 3.

After welding, specimens (of width 15 mm) were cut from the test plates (Figure 1), which were used to measure dimensions and hardness of the weld beads, as well as for bend testing. The measured cross-sectional dimensions of the weld beads are shown in Figure 2 and measured values (bead width, reinforcement height and penetration depth) are given in Table 4.

Based on the measured weld bead dimensions, the weld penetration shape factor is:

Dilution—the area of base metal melted in relation to the cross-sectional area of the bead (Figure 2) was determined using the formula:

The hardness of the weld bead reinforcement area was measured, as well as the heat-affected zones (HAZ) hardness on different distances from the hardfaced surface (Table 5). The hardness distribution in HAZ is shown by the diagram in Figure 3.

The hardness of HAZ is high near to the bead, decreasing under the hardness of the BM, on the depth of approx. (0.2–0.5) mm (Figure 3).

The scheme of the bend test is shown in Figure 4. The weld bead is in the tension zone of the bent specimen. During the test, each specimen was bent until the first crack appeared on the weld bead. At that moment, the bending angle $\alpha$ (Figure 4) was registered (Table 6).

Under the applied experimental conditions, the relatively large mass of the test plates compared to the deposited bead, together with the high proportion of BM in the fused zone, resulted in high bead hardness and limited plasticity. When higher arc energy and heat input were applied, lower bead hardness and larger bending angles at the onset of cracking were observed [10], [11], [12].

On the basis of the results presented in Table 4, Table 5, Table 6, the following observations can be made:

$\bullet$ Under all applied regimes, the reinforcement height exceeded the penetration depth into the BM;

$\bullet$ With increasing arc energy and heat input, the bead width increased accordingly;

$\bullet$ For flux-cored wire (regimes 9 and 10), smaller penetration depths were obtained at comparable arc energies and heat inputs than for gas-shielded wires, resulting in higher values of the weld penetration shape factor (WPSF);

$\bullet$ Although penetration depth generally increased with arc energy and heat input, no clear relationship was found between the penetration area $A_P$ and the total bead cross-sectional area $A_R$;

$\bullet$ The bending angle at the onset of cracking increased with arc energy, heat input, and bead width, but decreased with increasing bead hardness.

The hardness distribution in the HAZ is shown in Figure 3. Under the applied welding conditions, relatively high hardness values were measured close to the bead, followed by a decrease at depths of approximately 0.2–0.5 mm to values below the original BM hardness [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26].

The results show that the mechanical characteristics of the weld bead and the heat-affected zone in hardfaced 30CrMoV9 steel are strongly influenced by the choice of filler metal and the applied arc energy (i.e., heat input). For precision mechanical components made of this steel, which require high resistance to cracking, preheating and/or post-weld heat treatment is therefore necessary. The present study provides experimental data that can support the selection of welding parameters and filler materials for hardfacing applications involving 30CrMoV9 steel.