Tempering Effect of Subsequent Weld Passes on Previous Passes During Single-Layer Surfacing of 15CrMo Steel

Literature Overview

The paper published in Hot Working Technology (2021, Vol. 50, No. 17, pp. 122-125) by Feng Linjie, Liu Fuguang, Zhang Wei, Zhao Yongming, Ge Jun, Han Tianpeng, and Yang Erjuan investigates the thermal interaction between adjacent weld passes during single-layer surfacing of 15CrMo steel. The study employs a three-dimensional finite element model to simulate the temperature field evolution during the surfacing process and quantifies the tempering effect that subsequent passes exert on previously deposited passes. This research addresses a fundamental metallurgical question in multi-pass surfacing: how much post-weld tempering does each pass receive from subsequent passes, and is this tempering sufficient to prevent cracking in the high-carbon equivalent 15CrMo steel?

The Metallurgical Problem

15CrMo steel is a low-alloy steel widely used in power generation equipment, particularly in high-pressure boiler tubes, steam headers, and pressure vessel components. The steel has a chemical composition of approximately 0.15% C, 0.9% Cr, and 0.25% Mo, giving it a carbon equivalent (CE) of approximately 0.45-0.50%. This relatively high CE makes the steel susceptible to cold cracking during welding if proper pre-heating and interpass temperature control are not maintained. Surfacing operations on 15CrMo components often require a single layer of austenitic stainless steel overlay to provide corrosion resistance, and the surfacing process involves multiple passes to build up the required overlay thickness.

The tempering effect of subsequent passes is significant because it can reduce the hardness of previously deposited passes from the as-welded martensitic condition to a more tempered condition, thereby reducing the risk of hydrogen-induced cracking. Understanding the extent of this tempering effect is essential for determining whether additional post-weld heat treatment (PWHT) is necessary or whether the in-process tempering from subsequent passes is sufficient.

Finite Element Model Parameters

Key Findings from the Finite Element Analysis

The finite element simulation revealed that under identical preheat temperature, welding parameters, and material conditions, the temperature distributions of the three weld passes during welding are essentially the same. This finding indicates that the thermal history of each pass is dominated by its own welding heat input rather than by the thermal state of previously deposited material, at least in the immediate vicinity of the weld.

The most important finding is that the effective tempering zone width on each side of the weld is 3.6-4.6 mm. With a typical weld bead width of 10-12 mm for single-layer surfacing, this means that the tempering effect extends only about 25% of the previous pass width from each side. Consequently, the first subsequent pass tempers approximately 50% of the previous pass (25% from each side), leaving about 50% of the previous pass in the as-welded condition. Only when the overlap between passes reaches 75% does the tempering effect become complete, as the subsequent pass covers the entire width of the previous pass.

This finding has direct implications for surfacing procedure design. If the overlap between passes is only 50%, as is common practice, then the center of each pass remains in the untempered as-welded condition, with hardness potentially exceeding 400 HV. For 15CrMo steel, this hardness level is associated with an increased risk of hydrogen-induced cracking, particularly if the hydrogen content in the weld metal is not adequately controlled.

Engineering Practice Implications

The findings of this study suggest several practical recommendations for surfacing 15CrMo steel. First, the overlap between adjacent passes should be increased to at least 75% to ensure complete tempering of each pass by the subsequent pass. Second, if a 50% overlap is maintained for practical reasons, then post-weld heat treatment should be applied to the entire surfacing layer to achieve uniform tempering. Third, hydrogen control measures such as electrode baking, low-hydrogen filler metal selection, and thorough joint cleaning should be implemented to minimize the cracking risk in the untempered regions.

The preheat temperature of 150 °C used in the simulation is consistent with standard practice for 15CrMo steel welding. However, the study suggests that this preheat temperature primarily affects the base material rather than the deposited weld metal. The base material is heated to 150 °C to reduce the cooling rate and minimize the formation of hard martensite in the heat-affected zone. The weld metal itself is deposited from a molten pool and cools rapidly, so the preheat temperature has limited effect on the as-welded hardness of the deposit.

Study Insights and Reflections

This research provides valuable quantitative data on the tempering effect during multi-pass surfacing, which has been largely treated qualitatively in previous literature. The finite element approach allows precise determination of temperature distributions and tempering zone widths that would be difficult to measure experimentally due to the transient nature of the thermal events. The finding that 75% overlap is required for complete tempering is a significant practical insight that should influence surfacing procedure design for high-carbon equivalent steels. However, it is important to note that the finite element model assumes idealized conditions and does not account for factors such as wind cooling, surface oxidation, or variations in heat input that occur in actual field conditions. Validation of the model predictions through thermocouple measurements and hardness profiling of actual welds is recommended before applying the findings to production welding procedures.