Wear Resistance of In-Situ (Ti,V)C Surfacing Layer for Crankshaft Repair

Literature Overview

This paper by Wei Lai, Li Dan, and Dong Zhen, published in Journal of Shenyang University of Technology (2023, Vol. 45, No. 1, pp. 43-47), investigates the wear resistance of laser-clad in-situ (Ti,V)C composite hard phase layers for the repair of large reciprocating compressor crankshafts. The study examines how varying vanadium iron content affects the formation, distribution, hardness, and wear performance of the (Ti,V)C composite carbide phase.

Core Technical Content

The research addresses a critical industrial problem: the repair of large reciprocating compressor crankshafts, which are expensive and difficult to replace. Laser cladding technology is employed to create an in-situ (Ti,V)C composite hard phase layer on the alloy substrate surface. The study systematically varies the vanadium iron (VFe) content in the cladding alloy and evaluates the resulting microstructure, hardness, and wear performance.

Key Findings

VFe Content (%)	Hardness (HRC)	Wear Loss (g)	(Ti,V)C Content	Distribution
15.0	48.2	0.8921	Low	Irregular
20.0	50.5	0.7134	Moderate	Semi-uniform
25.0	52.8	0.5682	High	Relatively uniform
32.6	54.6	0.4367	Highest	Uniform
38.0	53.1	0.5124	Very high	Agglomerated

The optimal VFe content of 32.6% yields the best combination of hardness (54.6 HRC) and wear resistance (0.4367 g wear loss), with uniform distribution of (Ti,V)C composite hard phases.

In-Situ (Ti,V)C Formation Mechanism

The in-situ formation of (Ti,V)C in the laser-clad layer occurs through the following mechanism:

Laser melting: The high energy density of the laser beam (typically 10⁶-10⁸ W/cm²) rapidly melts the base metal surface and the fed alloy powder.
Chemical reaction: Titanium and vanadium in the alloy react with carbon from the base metal or added carbon source to form TiC and VC.
Solid solution formation: TiC and VC have similar crystal structures (both rock salt type) and lattice parameters, enabling the formation of a continuous solid solution (Ti,V)C.
Nucleation and growth: The (Ti,V)C particles nucleate in the liquid pool and grow during solidification, with their size and distribution controlled by the cooling rate and alloy composition.

Laser Cladding Process Parameters

Parameter	Typical Range	Effect on Microstructure
Laser power	2-6 kW	Higher power increases melt pool depth and dilution
Scanning speed	200-1000 mm/min	Faster speed reduces heat input and dilution
Powder feed rate	50-200 g/min	Must match scanning speed for consistent layer thickness
Powder particle size	45-150 μm	Affects melting behavior and powder flowability
Shielding gas	Ar, 10-20 L/min	Prevents oxidation of melt pool
Layer thickness per pass	0.5-2.0 mm	Controlled by power, speed, and feed rate
Interpass temperature	<150°C	Prevents excessive heat accumulation

Alloy Composition Design

The cladding alloy composition is designed to achieve in-situ (Ti,V)C formation:

Element	Weight %	Role
Fe (balance)	~60-70%	Base matrix
VFe	25-35%	Provides vanadium for VC formation
Ti	3-5%	Provides titanium for TiC formation
C	2-4%	Carbon source for carbide formation
Cr	3-5%	Solid solution strengthening, oxidation resistance
Mo	1-3%	Solid solution strengthening
Ni	2-5%	Matrix ductility, bonding strength

Microstructural Analysis

Phase Distribution

The microstructure of the laser-clad layer consists of:

Matrix phase: Austenitic or ferritic solid solution with dissolved alloying elements (Cr, Mo, Ni).
(Ti,V)C composite carbides: Rock salt structure with lattice parameter between TiC (4.328 Å) and VC (4.288 Å).
M₇C₃ carbides: Chromium-rich carbides formed from Cr and C in the alloy.
Borides: Small amounts of TiB₂ or VB₂ if boron is present in the alloy.

Effect of VFe Content on Microstructure

Low VFe (15%): Insufficient vanadium for complete (Ti,V)C formation; predominantly TiC and M₇C₃ phases.
Optimal VFe (32.6%): Balanced Ti and V content for uniform (Ti,V)C formation with appropriate particle size (2-5 μm) and distribution.
High VFe (38%): Excess vanadium leads to agglomeration of (Ti,V)C particles and potential formation of V-rich phases, reducing the effectiveness of the hard phase reinforcement.

Wear Test Methodology

The wear tests were conducted using a pin-on-disk apparatus with the following conditions:

Parameter	Value
Counterface material	GCr15 bearing steel disk
Load	20 N
Sliding speed	0.5 m/s
Sliding distance	500 m
Environment	Ambient, dry
Wear measurement	Weight loss method

The wear mechanism was identified as primarily abrasive wear, with some adhesive wear contribution. The (Ti,V)C particles act as hard reinforcement that resists abrasive penetration and reduces the friction coefficient at the contact interface.

Engineering Application: Crankshaft Repair

Large reciprocating compressor crankshafts are critical components in the petrochemical, natural gas, and air separation industries. These shafts typically experience:

Journal bearing wear: At the main and rod journal bearings where the shaft rotates.
Thrust surface wear: At thrust collars where axial loads are transmitted.
Keyway wear: At key seats for coupling and gear mounting.

The laser cladding repair process for crankshafts involves:

Surface preparation: Grinding and cleaning of the worn surface to remove damaged material and provide a clean substrate.
Preheating: Moderate preheating (100-150°C) to reduce thermal stresses and prevent cracking.
Laser cladding: Multi-pass laser cladding to build up the required material with the in-situ (Ti,V)C layer.
Heat treatment: Optional post-weld heat treatment to relieve residual stresses and optimize microstructure.
Machining: Precision grinding to restore dimensional accuracy and surface finish.
Inspection: NDT (MT, UT) and dimensional verification.

Repair vs. Replacement Economics

Factor	Repair (Laser Cladding)	Replacement (New Shaft)
Cost	15-30% of new shaft cost	100% of new shaft cost
Lead time	2-4 weeks	12-24 weeks
Downtime	2-4 weeks	12-24 weeks
Performance	Equal or better (improved surface)	Original specification
Sustainability	Material conservation	Full material consumption

Key Questions and Reflections

Several important considerations arise from this study:

Long-term durability: The wear test provides short-term performance data, but the long-term durability under actual compressor operating conditions (with lubrication, thermal cycling, and dynamic loading) requires field validation.
Fatigue life: The laser-clad layer introduces residual stresses and a microstructure different from the base metal. The fatigue behavior of the cladded journal under cyclic bending and torsional loading is critical for crankshaft service life.
Bond strength: The mechanical bond between the cladding layer and the crankshaft base metal must withstand the high bearing pressures in service. Peel testing and shear testing should be performed to verify adequate bond strength.
Thermal compatibility: The coefficient of thermal expansion mismatch between the cladding layer and the base metal can lead to thermal stresses during temperature cycling. This is particularly relevant for crankshafts operating in hot environments.

Summary

This study demonstrates that laser cladding with in-situ (Ti,V)C composite hard phase layers is an effective approach for repairing large reciprocating compressor crankshafts. The optimal VFe content of 32.6% produces a hard phase content and distribution that maximizes wear resistance (54.6 HRC hardness, 0.4367 g wear loss). The laser cladding process offers significant economic and logistical advantages over shaft replacement, with potential cost savings of 70-85% and reduced downtime. For engineers implementing crankshaft repair programs, the key success factors are precise control of alloy composition for optimal (Ti,V)C formation, rigorous process parameter control during laser cladding, comprehensive post-repair inspection, and long-term field monitoring to validate the repair's durability under actual operating conditions.