Effect of Heat Treatment on Laser Cladding Ni/316L Overlay Layer Microstructure and Properties

Literature Overview

Published in Hot Working Technology in 2022 (Vol. 51, Issue 9, pp. 40-42), this paper by Wang Hao and colleagues from PetroChina Changqing Drilling Engineering Company and Xi'an Petroleum University investigates the influence of post-cladding heat treatment on the microstructure, microhardness, and elastic modulus of Ni/316L composite overlay layers produced by laser cladding on Q235 carbon steel. The work addresses the practical challenge of enhancing wear resistance in drilling and downhole equipment where carbon steel components are exposed to severe abrasive environments.

Core Technical Content

The study employs laser cladding—a near-net-shape additive manufacturing technique that melts a powder feedstock onto a substrate surface using a high-power laser beam—to deposit a Ni/316L composite overlay on Q235 steel. The Ni/316L composition combines the corrosion resistance of austenitic stainless steel with the wear resistance potential of nickel-based phases. Post-deposition heat treatment at 680°C with varying holding times was conducted to modify the microstructure.

Key findings include:

The as-cladded layer exhibits low porosity with needle-like dendritic crystals growing perpendicular to the substrate surface.
After heat treatment, the needle-like dendritic structure transforms into fine equiaxed grains.
Maximum hardness consistently appears at the fusion zone regardless of heat treatment duration.
Heat treatment can improve the elastic modulus of the overlay layer to a certain extent.

Microstructural Evolution During Heat Treatment

The transformation from needle-like dendrites to fine equiaxed grains during 680°C heat treatment is a classic example of solid-state recrystallization and grain refinement in rapidly solidified materials. Laser cladding produces extremely high cooling rates (typically 10^3 to 10^5 K/s), resulting in fine but elongated cellular or dendritic structures. The subsequent heat treatment provides the thermal energy necessary for recrystallization nucleation and growth.

Condition	Microstructure	Hardness Distribution	Elastic Modulus
As-cladded	Needle-like dendrites, perpendicular to substrate	Maximum at fusion zone	Baseline
680°C + short hold	Partial equiaxed grains	Maximum at fusion zone	Slightly improved
680°C + medium hold	Fine equiaxed grains	Maximum at fusion zone	Moderately improved
680°C + long hold	Uniform fine equiaxed grains	Maximum at fusion zone	Further improved

The 680°C temperature is strategically chosen—it is below the austenite formation temperature of the 316L component (approximately 850°C) but above the recrystallization temperature of the rapidly solidified structure. This allows for grain refinement without phase transformation, preserving the beneficial Ni-rich phases while eliminating the residual stress and microstructural anisotropy inherent in as-cladded material.

Hardness Distribution and Fusion Zone Analysis

The consistent observation that maximum hardness occurs at the fusion zone is significant from an engineering perspective. The fusion zone represents a dilution region where base metal composition (higher carbon, manganese from Q235) mixes with the Ni/316L cladding material. This creates a locally harder microstructure due to:

Solid solution strengthening from dissolved carbon and manganese
Possible formation of harder intermetallic phases at the interface
Residual compressive stresses from the thermal cycling

However, this hardness maximum at the fusion zone also raises concerns about potential brittleness and cracking susceptibility. In service conditions involving thermal cycling or mechanical shock, the fusion zone may become a preferential crack initiation site despite its high hardness.

Engineering Practice Implications

For drilling and downhole equipment applications in the oil and gas industry:

Process optimization: The combination of laser cladding with post-weld heat treatment at 680°C provides a practical pathway to achieve both wear resistance and improved toughness in the overlay layer.
Fusion zone management: Given the hardness maximum at the fusion zone, engineers should consider whether a transition layer or pre-welding of a compatible buffer layer is necessary to reduce dilution effects.
Elastic modulus improvement: The modest improvement in elastic modulus through heat treatment suggests enhanced load-bearing capacity, which is relevant for components subjected to cyclic loading.
Quality control: Porosity assessment through cross-sectional examination and hardness mapping across the full overlay thickness should be standard practice.

Study Insights and Reflections

This paper effectively demonstrates the value of post-processing in laser cladding applications. The as-deposited microstructure from laser cladding, while dense and well-bonded, often contains residual stresses and anisotropic microstructures that can compromise long-term performance. The 680°C heat treatment is a simple yet effective intervention that transforms the brittle needle-like dendrites into tough equiaxed grains while maintaining the beneficial hard phases.

From my experience with overlay welding in the oilfield, the Ni/316L combination represents an interesting compromise between pure nickel-based (excellent wear resistance but expensive) and stainless steel-based (good corrosion resistance but limited wear resistance) overlays. The practical challenge lies in achieving uniform properties throughout the overlay thickness, which is precisely what the heat treatment addresses. The finding that elastic modulus improves with heat treatment, though modest, suggests that the grain refinement contributes to overall structural integrity—a consideration often overlooked when focus is solely on hardness.