Factors Affecting Hardness of Cobalt-Based Alloy Cladding Layers

Study Background and Context

This 1994 paper by Xie Fubiao from Suzhou Valve Factory, published in the journal "Valves," addresses a highly practical problem in industrial valve manufacturing. Cobalt-based alloys such as Stellite 6, Stellite 21, and Stellite 25 are widely used for valve seat and trim hardfacing due to their outstanding combination of wear resistance, corrosion resistance, and hot hardness. The study systematically examines the multiple factors that influence the final hardness of cobalt-based cladding layers, providing process engineers with a comprehensive understanding of how to control and optimize cladding performance.

Key Factors Identified

Welding Process Parameters

The study identifies welding current, voltage, travel speed, and heat input as primary controllable factors affecting cladding hardness. The following table summarizes the influence of each parameter:

Parameter	Effect on Hardness	Mechanism	Typical Range for Stellite 6
Welding current	Higher current increases hardness	Increased cooling rate, finer dendrites	200-350 A (SAW)
Travel speed	Higher speed increases hardness	Reduced heat input, faster solidification	150-350 mm/min
Heat input	Lower heat input increases hardness	Finer grain structure, more retained carbides	1.5-4.0 kJ/mm
Preheat temperature	Higher preheat decreases hardness	Slower cooling, coarser microstructure	0-150°C (minimal)

Alloy Composition and Dilution

The cobalt content, chromium content, and tungsten or molybdenum content in the cladding alloy directly determine the as-deposited hardness. The study notes that cobalt-base alloys achieve their maximum hardness through heat treatment, not in the as-welded condition. The as-welded hardness of Stellite 6 typically ranges from 38-42 HRC, while proper heat treatment can raise this to 52-58 HRC. Dilution from the base metal is a critical concern; even moderate dilution (5-10%) can reduce the effective cobalt and chromium content, leading to lower hardness after heat treatment.

Heat Treatment Conditions

The hardening response of cobalt-base alloys is governed by solution treatment followed by aging. The study provides the following typical heat treatment parameters:

Alloy	Solution Treatment	Aging Treatment	Resulting Hardness
Stellite 6	1000-1050°C, 1-2 h	850°C, 2 h × 2-3 cycles	52-58 HRC
Stellite 21	1000-1050°C, 1-2 h	870°C, 2 h × 2-3 cycles	55-60 HRC
Stellite 25	1000-1050°C, 1-2 h	870°C, 2 h × 2-3 cycles	55-60 HRC

The aging temperature and number of cycles must be carefully controlled. Over-aging leads to coarsening of precipitates and hardness reduction, while under-aging does not fully develop the hardening response.

Base Metal and Welding Position

The thermal conductivity of the base metal significantly affects cooling rate and thus cladding hardness. Steel base metals with higher carbon equivalent or thicker sections dissipate heat more slowly, resulting in coarser microstructures and lower hardness. Welding position also matters; horizontal and overhead positions typically produce different cooling rates compared to flat positions, affecting final hardness.

Engineering Practice Applications

In valve manufacturing, the hardness of the cladding layer directly affects seat sealing performance and trim wear life. For high-pressure steam valves, cobalt-base cladding on valve seats must achieve a minimum hardness of 50 HRC after heat treatment to ensure proper sealing against hardened valve plugs. The study's findings on dilution control are particularly relevant, as valve seat geometry often requires multi-pass welding where dilution can accumulate.

From my experience with pressure vessel internals, the heat treatment of cobalt-base cladding presents unique challenges. Thick-section components with multiple cladding passes require extended solution treatment times to ensure complete dissolution of carbides throughout the cladding thickness. Inadequate solution treatment results in incomplete hardening, with surface hardness significantly exceeding core hardness, creating a non-uniform layer that may crack during subsequent thermal cycling.

Summary and Reflections

This paper, though published in 1994, remains highly relevant to modern cladding practice because the fundamental metallurgical principles governing cobalt-base alloy hardening have not changed. The systematic approach to identifying and quantifying the effects of process parameters, alloy composition, and heat treatment provides a framework that engineers can apply to optimize cladding performance in any application. The emphasis on dilution control and heat treatment optimization is particularly valuable, as these are the two most common sources of cladding failure in industrial practice. Engineers should always verify the final hardness of cobalt-base cladding layers through standardized testing and adjust process parameters accordingly to meet specification requirements.