Research on Weld Overlay Process for Crankshaft Repair

Literature Overview

This 2020 publication in the "Journal of Shenyang Ligong University" (沈阳理工大学学报) by Lei Jinhong, Zhou Qiuzhong, Chen Zhuojun, Cui Liang, and Gu Xiaolong, supported by the Liaoning Provincial Department of Education Key Research Project (LG201901) and the Shenyang Municipal Middle-aged and Young Scientific and Technological Innovation Talent Support Program, presents a systematic study of weld overlay processes for crankshaft repair. Crankshafts are among the most critical components in internal combustion engines, and their repair through weld overlay is a well-established practice in the automotive and marine engineering industries. The study addresses the unique challenges of crankshaft repair, including the restoration of journal dimensions, the maintenance of material properties in the heat-affected zone, and the control of residual stress to prevent subsequent fatigue failure.

Technical Challenges of Crankshaft Repair

The crankshaft is a highly stressed component that undergoes complex loading conditions including bending, torsion, and fatigue. The repair of worn or damaged journals through weld overlay requires addressing several technical challenges:

Challenge	Description	Technical Requirement
Dimensional restoration	Worn journals must be built up to oversize dimensions	Controlled deposition rate and geometry
Material compatibility	Overlay alloy must match or exceed base material properties	Similar thermal expansion, compatible metallurgy
HAZ integrity	Heat input must not soften or embrittle the base material	Low heat input, preheating, interpass temperature control
Residual stress control	Welding residual stress must not initiate fatigue cracks	Stress relief heat treatment, welding sequence optimization
Surface finish	Final journal surface must meet tight tolerance requirements	Post-weld machining to Ra ≤ 0.8 μm
Fatigue resistance	Repaired journal must withstand cyclic loading	Proper heat treatment, smooth transition at weld toe

Process Comparison and Selection

The study compares several welding processes for crankshaft journal repair, evaluating their suitability based on deposition rate, heat input, distortion control, and final surface quality:

Process	Deposition Rate (g/min)	Heat Input (kJ/mm)	Distortion Control	Surface Quality	Suitability
GTAW (TIG)	10–25	1.0–2.0	Excellent	Excellent	Best for small repairs
GMAW (MIG)	30–60	2.0–3.5	Good	Good	Moderate repairs
SAW (Submerged Arc)	80–150	3.0–5.0	Fair	Poor (requires machining)	Large build-up
Laser Cladding	20–50	0.5–1.5	Excellent	Excellent	Precision repair
Hot-Wire TIG	40–80	1.5–3.0	Good	Good	Balanced approach
FCAW	40–70	2.0–4.0	Fair	Fair	Cost-effective for large repairs

Based on the comparison, the study recommends a hybrid approach: GTAW or hot-wire TIG for the initial passes to establish a controlled fusion boundary with minimal dilution, followed by GMAW or FCAW for the bulk deposition to improve productivity, and finishing with GTAW for the final surface pass to achieve the required surface quality.

Detailed Process Parameters for Crankshaft Journal Repair

The recommended welding parameters for the repair of a typical medium-duty engine crankshaft journal are as follows:

Parameter	Value	Remarks
Base material	42CrMo or 40CrNiMo	Typical crankshaft alloy steel
Overlay wire	ER80S-D2 or equivalent	Matching or slightly higher strength
Preheat temperature	150–250 °C	To prevent cracking and reduce HAZ hardness
Interpass temperature	≤ 250 °C	To avoid excessive grain growth
GTAW current	80–120 A	AC or DCEN, depending on wire type
GTAW travel speed	30–60 mm/min	For controlled bead geometry
GMAW current	180–250 A	Short-circuit transfer mode
GMAW voltage	18–22 V	For stable arc and good wetting
Shielding gas	Ar + 5–8% CO₂	For GMAW; pure Ar for GTAW
Post-weld heat treatment	580–620 °C, 2–4 h	Stress relief to reduce residual stress
Final hardness target	28–35 HRC	Matching original journal hardness

The preheating step is critical for crankshaft repair because the alloy steel base material has a relatively high carbon equivalent (Ceq ≈ 0.45–0.55), which increases the susceptibility to cold cracking. Preheating at 150–250 °C reduces the cooling rate of the weld, allowing hydrogen to diffuse out of the weld metal and reducing the risk of hydrogen-induced cracking.

Defect Analysis and Countermeasures

The study identifies the following common defects and their countermeasures:

Defect	Root Cause	Detection Method	Countermeasure
Cracking in overlay	High cooling rate, hydrogen presence	MT, UT	Preheat, low heat input, post-weld bake
Excessive HAZ hardness	High heat input, rapid cooling	Hardness survey	Reduce heat input, increase preheat
Porosity	Gas absorption, contamination	RT, UT	Clean surfaces, dry flux, proper shielding
Incomplete fusion	Low current, slow travel speed	UT, MT	Increase current, optimize travel speed
Distortion	Asymmetric heat input	Measurement	Balanced welding sequence, fixture support
Hardness mismatch	Inadequate heat treatment	Hardness test	Proper stress relief cycle

A particularly important aspect of the study is the emphasis on the transition zone between the overlay and the base material. The hardness gradient at this interface must be smooth and gradual to avoid stress concentrations that could initiate fatigue cracks during engine operation. The study recommends that the hardness transition from the overlay to the base material should not exceed 5 HRC over a distance of 1 mm, which can be achieved through proper control of the dilution ratio and the post-weld heat treatment cycle.

Study Insights and Engineering Implications

This study provides a comprehensive framework for crankshaft repair through weld overlay, addressing the full spectrum of technical considerations from process selection to defect prevention. The key insight is that crankshaft repair is not a simple build-up operation; it is a metallurgical challenge that requires careful balancing of deposition rate, heat input, residual stress, and final mechanical properties. The recommendation of a hybrid welding approach, combining different processes for different stages of the repair, reflects a practical engineering philosophy of using the right tool for the right job. For maintenance engineers, the study also highlights the importance of post-repair inspection and qualification testing, including dimensional measurement, hardness survey, and non-destructive testing, to ensure that the repaired journal meets the required specifications before returning to service. The economic and environmental benefits of repairing rather than replacing crankshafts are significant, particularly for large marine engines where a single crankshaft can weigh several tons and cost hundreds of thousands of dollars. This study contributes to the body of knowledge that enables engineers to make informed decisions about the feasibility and reliability of crankshaft repair.