Influence of Heat Treatment on Microstructure and Properties of Laser Cladded Ni-316L Overlay Layer

Literature Overview

The paper under review investigates the effect of post-weld heat treatment (PWHT) on the microstructure, mechanical properties, and corrosion resistance of a Ni/316L bimetallic overlay layer produced by laser cladding on a carbon steel substrate. This topic is of considerable practical importance because nickel-based and stainless steel overlay layers are widely deployed in chemical processing, oil and gas, and power generation equipment where combined wear-corrosion resistance and metallurgical bonding with ferrous substrates are required. The study systematically compares the as-cladded condition with several heat treatment regimes, including solution annealing at 1050 °C for 1 hour followed by water quenching, and aging treatments at 700 °C and 850 °C for 2 hours.

Core Technical Findings

The as-cladded overlay layer exhibits a columnar dendritic microstructure with interdendritic segregation of chromium and molybdenum, which is characteristic of the rapid solidification rates inherent to laser cladding (typically 10^4 to 10^6 K/s). The presence of Cr23C6 and Mo2C carbides at the interdendritic regions contributes to localized galvanic cells that can accelerate intergranular corrosion attack. The heat treatment at 1050 °C dissolves these carbides and homogenizes the dendritic composition, resulting in a finer equiaxed grain structure. Subsequent aging at 700 °C promotes the precipitation of fine Ni3(Fe,Cr) intermetallic particles that provide age-hardening without significantly degrading ductility.

Condition	Grain Structure	Hardness (HV)	Intergranular Corrosion Test Result	Bond Strength (MPa)
As-cladded	Columnar dendritic	285–310	Failed (ASTM A923 Practice E)	245
Solution anneal 1050 °C/1h + WQ	Equiaxed, fine	220–245	Passed	260
Solution + Aging 700 °C/2h	Equiaxed with fine precipitates	265–290	Passed	255
Solution + Aging 850 °C/2h	Coarsened precipitates	210–230	Passed	250

The solution annealing step is critical for eliminating the sensitization caused by carbide precipitation at the grain boundaries. However, excessive aging temperatures (above 850 °C) lead to precipitate coarsening and a decline in hardness, which reduces the wear resistance advantage of the overlay.

Process Parameters and Heat Treatment Windows

The laser cladding process was conducted using a 4 kW fiber laser with a scan speed of 1.2 m/min, powder feed rate of 15 g/min, and a Ni-based alloy powder with 316L content of approximately 40 wt%. The dilution ratio from the substrate was measured at 8–12%, which is acceptable for maintaining the corrosion resistance of the overlay. The heat treatment parameters were selected based on the TTT diagram of the Ni-Cr-Fe system and the sensitization behavior of austenitic stainless steels.

Parameter	Value / Range
Laser power	3.5–4.5 kW
Scan speed	0.8–1.5 m/min
Powder feed rate	12–18 g/min
Substrate preheat	150–200 °C
Interpass temperature	< 300 °C
Solution annealing	1050 °C, 1 h, water quench
Aging	700 °C or 850 °C, 2 h, air cool

The preheat temperature is kept moderate to avoid excessive thermal input that could dilute the overlay with the carbon steel substrate. The interpass temperature limit of 300 °C ensures that each successive layer solidifies with sufficient cooling rate to maintain a fine microstructure.

Defect Analysis and Countermeasures

During the cladding process, porosity and lack of fusion were the most frequently observed defects. Porosity, predominantly gas porosity from hydrogen and nitrogen absorption, was reduced by increasing the shielding gas flow rate to 25 L/min and using high-purity argon. Lack of fusion at the interface between the overlay and substrate was addressed by optimizing the overlap ratio to 50–60% and ensuring proper surface preparation (grinding to bare metal with 120-grit abrasive). After heat treatment, a minor risk of hot cracking at the overlay-substrate interface exists due to the thermal expansion mismatch between the Ni-based overlay and carbon steel substrate; this was mitigated by controlling the cooling rate during quenching to not exceed 50 °C/min in the critical temperature range of 400–600 °C.

Engineering Practice Implications

From an engineering perspective, the key takeaway is that laser cladding of Ni/316L overlays on carbon steel substrates requires a carefully designed post-weld heat treatment cycle to achieve both corrosion resistance and mechanical integrity. The as-cladded condition, while offering high hardness, is susceptible to intergranular corrosion due to carbide segregation. The recommended practice is a two-step heat treatment: solution annealing followed by aging at 700 °C. This combination provides a balanced set of properties suitable for applications such as heat exchanger tubes, pump shafts, and valve seats operating in chloride-containing environments. The bond strength exceeding 240 MPa confirms that the metallurgical bond is robust and not compromised by the heat treatment, which is essential for pressure vessel and rotating equipment applications.

Summary

The study demonstrates that post-weld heat treatment is not merely an optional step but a mandatory requirement for laser-cladded Ni/316L overlays intended for corrosion-critical service. The solution annealing at 1050 °C eliminates sensitization, while aging at 700 °C restores hardness without re-introducing carbide precipitation. Engineers designing cladding specifications should mandate a documented heat treatment cycle and verify intergranular corrosion resistance through standardized testing before commissioning the equipment. This approach aligns with the quality assurance philosophy of preventive control rather than post-failure remediation, and it provides a clear process window that can be incorporated into fabrication procedures and inspection plans.