Cold Hardfacing of Wear-Resistant Layer on Inner Surface of Concrete Distribution Valve Shell

Literature Overview

This study note examines the cold hardfacing of a wear-resistant layer on the inner surface of a concrete distribution valve shell, reported by Ma Caiwang from Hubei Construction Machinery Factory in 1991. Concrete distribution valves are critical components in concrete pumping systems, subjected to severe abrasive wear from concrete containing aggregates. The inner surface of the valve shell experiences direct contact with flowing concrete, leading to rapid wear and valve failure. Cold hardfacing, typically referring to surfacing processes performed without preheating or with minimal preheating, provides an effective means of extending valve shell life. This literature reflects an early application of cold hardfacing technology in China's construction machinery industry.

Core Technical Content

Service Conditions of Concrete Distribution Valves

Concrete distribution valves operate under the following conditions:

Abrasive wear: Contact with concrete containing sand, gravel, and other aggregates.
Impact loading: Concrete particles impact the valve surface at high velocity.
Cyclic loading: Repeated opening and closing cycles during pumping operations.
Corrosive environment: Exposure to alkaline concrete (pH 12–14) and water.
Temperature variations: From ambient to elevated temperatures during pumping.

The valve shell inner surface must resist all these degradation mechanisms simultaneously.

Cold Hardfacing Process Definition

Cold hardfacing refers to surfacing processes performed without preheating or with minimal preheating (typically below 100 °C). This is in contrast to hot hardfacing, which involves preheating to 200–400 °C or higher. Cold hardfacing is suitable for:

Thin-walled components where preheating may cause distortion.
Components with tight dimensional tolerances.
Applications where minimal heat input is required.
Field repair and maintenance operations.

Common cold hardfacing processes include:

Cold spray: High-velocity particle deposition with minimal heat input.
Low-heat-input arc surfacing: Using processes with low heat input such as TIG or PTA.
Flame spraying: Without preheating, using low-temperature flame settings.
Brush welding: Manual arc surfacing with controlled heat input.

Material Selection for Concrete Valve Hardfacing

Alloy Type	Key Elements	Hardness	Wear Resistance	Application
High-carbon martensite	C, Cr, Mo	55–60 HRC	High	General wear resistance
Carbide-forming alloy	C, Cr, W, Mo	60–65 HRC	Very high	Severe abrasive wear
Austenitic alloy	Ni, Cr, Mn	30–40 HRC	Moderate	Toughness required
Stellite-type	Co, Cr, W, Mo	40–45 HRC	High	High temperature wear

For concrete distribution valves, high-carbon martensitic or carbide-forming alloys are commonly selected due to their excellent abrasive wear resistance. The hardfacing layer thickness is typically 1–3 mm, sufficient to provide protection while maintaining valve dimensional accuracy.

Process Parameters for Cold Hardfacing

Parameter	Typical Range	Notes
Preheat temperature	0–100 °C	Minimal preheating
Heat input	1.0–2.5 kJ/mm	Low heat input to minimize distortion
Travel speed	100–300 mm/min	Higher speed for lower heat input
Arc voltage	15–22 V	Controlled to maintain stable arc
Shielding gas	Argon or CO2	Provides inert atmosphere
Interpass temperature	≤ 150 °C	Prevents overheating
Post-weld treatment	Stress relief at 300–400 °C	Reduces residual stresses

Defect Analysis and Quality Control

Common Defects in Cold Hardfacing

Defect	Cause	Impact	Countermeasure
Cracking	High residual stress, low toughness	Valve failure, leakage	Reduce heat input, use compatible filler
Spalling	Poor bond strength, thermal fatigue	Surface damage, reduced wear life	Optimize microstructure, ensure good bond
Excessive dilution	High heat input, thin layer	Reduced hardness, poor wear resistance	Control heat input, use low-heat processes
Hardness variation	Uneven cooling, composition variation	Inconsistent performance	Optimize process parameters
Incomplete fusion	Poor joint preparation, low heat input	Weak bond, delamination	Ensure good joint fit, increase heat input if needed

Quality Control Procedures

The quality of cold hardfaced valve shells is ensured through:

Pre-weld inspection: Verification of base material, surface preparation, and dimensional accuracy.
In-process monitoring: Control of process parameters, visual inspection of weld appearance.
Post-weld testing:

Hardness mapping: Ensuring uniform hardness across the hardfaced surface.
Penetrant testing (PT): Detecting surface cracks and defects.
Ultrasonic testing (UT): Evaluating bond strength and internal defects.
Metallographic examination: Assessing microstructure and bond quality.

Performance testing: Wear testing under simulated concrete flow conditions.

Engineering Practice Insights

Application Examples

Cold hardfacing has been successfully applied to various concrete distribution valve components:

Valve shell inner surface: Providing wear resistance in the concrete flow path.
Valve seat: Enhancing wear resistance in the sealing area.
Guide surfaces: Improving wear resistance in the guide area.
Wear rings: Protecting against wear in the wear ring area.

Economic Considerations

The economic viability of cold hardfacing compared to replacement valve shells depends on:

Cost of base material: Carbon steel or low-alloy steel bases are significantly cheaper than solid wear-resistant alloys.
Overlay cost: Powder or wire cost, processing cost, and labor cost.
Service life extension: Hardfacing can extend valve shell life by 3–5 times compared to unhardfaced shells.
Maintenance cost: Reduced replacement frequency and downtime.

Key Reflections

The application of cold hardfacing to concrete distribution valve shells represents a practical solution to the severe abrasive wear problem in concrete pumping systems. The low heat input of cold hardfacing processes makes them suitable for thin-walled valve shells where preheating may cause distortion. However, the success of the process depends critically on careful process parameter optimization, thorough quality control, and appropriate material selection. Engineers must understand the specific service conditions and select overlay materials accordingly. The evolution from manual to automated hardfacing systems has further improved consistency and productivity, making hardfacing an increasingly attractive option for high-performance valve applications. The future of valve hardfacing lies in the development of advanced wear-resistant alloys combined with advanced hardfacing processes offering even greater control and precision. The integration of hardfacing into routine maintenance practices will further extend the service life of concrete pumping equipment, reducing overall costs and improving productivity.