Howat Group Engineering Case Study

Pre- and Post-Weld Machined Valve Bodies

Published: 10th November 2025 | Issue 102 Share article:

Background

Valve bodies are critical pressure-retaining components in oil & gas, petrochemical, and power generation systems. They are typically manufactured from forged or cast materials such as ASTM A105, A350 LF2, or A182 F316, and are subject to welding during fabrication (for example, to attach flanges, nozzles, or seats).

To achieve dimensional precision and ensure sealing integrity, machining operations are required. A key engineering decision involves whether machining should be performed before welding (pre-weld machining), after welding (post-weld machining), or in a combined sequence. 

Objective

To evaluate and compare the dimensional accuracy, structural integrity, and manufacturing efficiency between pre-weld and post-weld machined valve bodies.

Process Overview

Step

Pre-Weld Machining

Post-Weld Machining

1

Material cutting & rough machining of major faces, threads, and seats

Rough machining only (minimal)

2

Fit-up & welding of end connections, nozzles, and bosses

Welding with more allowance for machining

3

Post-weld heat treatment (PWHT) if required

PWHT before final machining

4

Finish machining of sealing faces and bores

Full finish machining after weld & PWHT

5

NDE (RT, UT, PT)

NDE after welding and machining

 

Case Example

Component: 6” class 600 globe valve body

Material: ASTM A105 (carbon steel)

Welding Process: GTAW + SMAW for root and fill passes

PWHT: 620°C for 2 hours

Scenario A – Pre-Weld Machined

  • Machined to 0.1mm tolerance before welding
  • Distortion of 1.5mm observed post-welding
  • Rework needed on seat bore and flange faces
  • Total machining time 10 hours (including rework)
  • Final inspection: Acceptable but with reduced efficiency

Scenario B – Post-Weld Machining

  • Welded in as-forged condition, 3mm machining allowance provided
  • PWHT performed before final machining
  • Distortion absorbed in final machining work
  • Total machining time: 8 hours (no rework)
  • Final inspection: Acceptable with high dimensional accuracy

Results Summary

Criteria

Pre-Weld Machined

Post-Weld Machined

Dimensional Accuracy

Moderate (distortion from welding)

High (corrected during final machining)

Machining Time

Higher (due to rework)

Lower overall

Cost

Higher (due to rework)

Lower

Risk of Weld Defects

Slightly Higher (heat near machined surfaces)

Lower (less contamination)

NDE Accessibility

Easier before welding

Easier after machining

Surface Integrity

Potentially degraded by weld heat

Preserved post-PWHT

Discussion

Pre-weld machining is useful when:

  • Precise alignment for welding fixtures is required
  • Material is difficult to machine after welding (e.g., work-hardening alloys)

However, post-weld machining is generally preferred for:

  • Maintaining tight dimensional tolerances
  • Eliminating distortion effects
  • Reducing rework time and costs

For high-pressure valves or critical service applications (e.g., hydrogen or cryogenic valves), post-weld machining ensures dimensional stability and better sealing surface integrity.

Engineering Recommendations

  • Provide sufficient machining allowance (typically 2-4 mm) for post-weld operations
  • Use controlled welding sequences and fixturing to minimize distortion
  • Perform PWHT before finish machining to relieve residual stresses
  • Conduct dimensional inspection after each critical step (fit-up, welding, PWHT, and final machining)
  • Document distortion trends for process improvement

Conclusion

This study demonstrates that post-weld machining of valve bodies offers superior results in terms of dimensional accuracy, cost efficiency, and surface integrity. While pre-weld machining may assist in alignment and fixturing, it’s susceptibility to weld-induced distortion makes it less efficient for precision components. 

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