In thermal power plants, steel casting components like high-pressure main steam valves are critical for operational efficiency. This article details the successful repair of cracks in ZG15Cr1Mo1V steel casting valves from a 300 MW supercritical coal-fired unit, demonstrating advanced welding techniques for cast steel restoration.
1. Material Characteristics and Failure Analysis
The ZG15Cr1Mo1V steel casting exhibits excellent high-temperature stability with the following composition:
| Element | C | Mn | Si | Cr | Mo | V |
|---|---|---|---|---|---|---|
| Content (wt%) | 0.12-0.20 | 0.40-0.70 | 0.20-0.60 | 1.20-1.70 | 0.90-1.20 | 0.25-0.40 |
Mechanical properties at room temperature:
| Property | Rm (MPa) | Re (MPa) | Elongation (%) |
|---|---|---|---|
| Value | ≥490 | ≥343 | ≥14 |
Crack propagation in steel casting components follows the Paris Law relationship:
$$ \frac{da}{dN} = C(\Delta K)^m $$
Where da/dN represents crack growth rate, ΔK is stress intensity factor range, and C/m are material constants.
2. Welding Repair Methodology
The ENiCrMo-3 nickel-based welding material was selected for its compatibility with ZG15Cr1Mo1V steel casting:
| Element | Ni | Cr | Mo | Nb+Ta |
|---|---|---|---|---|
| Content (wt%) | 64.8 | 21.59 | 8.65 | 3.654 |
Welding parameters were optimized through thermal analysis:
$$ Q = \eta \frac{VI}{v} $$
Where Q = heat input (J/mm), η = thermal efficiency (0.7-0.8 for TIG), V = voltage (V), I = current (A), and v = travel speed (mm/s).
| Wire Diameter (mm) | Current (A) | Voltage (V) | Travel Speed (mm/s) |
|---|---|---|---|
| 1.6 | 90-130 | 14-18 | 2.5-3.5 |
| 2.5 | 120-175 | 15-20 | 3.0-4.0 |
3. Residual Stress Management
Peening effectiveness was quantified using the Almen intensity formula:
$$ I = N \left(\frac{F}{A}\right)^{3/2} $$
Where I = peening intensity, N = number of impacts, F = impact force, and A = contact area.
4. Quality Verification
Post-repair inspection criteria for steel casting components:
| Test | Standard | Acceptance Criteria |
|---|---|---|
| PT | NB/T 47013 | No cracks >1.5mm |
| UT | GB/T 7233 | Class II or better |
| Hardness | ASTM E18 | 150-220 HB |
The successful repair of ZG15Cr1Mo1V steel casting components demonstrates that proper welding techniques can restore critical infrastructure while maintaining the integrity of original steel casting materials. This approach significantly extends service life while reducing replacement costs for large steel casting components in power generation systems.