Metallurgy & Heat Treatment
Preheating, interpass temperature, PWHT, carbon equivalent, HAZ hardness limits, joint types, backing strips, and welder qualification.
- Explain the purpose of preheating and interpass temperature control
- Describe post weld heat treatment and stress relief
- Use carbon equivalent to assess weldability
- Identify joint types and backing strip applications
- Understand welder qualification and back gouging
Leçon 1
Preheating & Interpass Temperature Control
Why Preheat?
Preheating before welding is done primarily to reduce cooling rate and prevent cracking. By raising the base metal temperature before welding, the rate of heat dissipation from the weld zone is slowed. This slower cooling rate reduces the formation of hard, brittle martensite in the HAZ and allows dissolved hydrogen more time to escape, both of which significantly reduce the risk of cracking.
Interpass Temperature
The interpass temperature is the temperature of the weld area between passes in a multipass weld. It is measured at the weld joint before depositing the next pass. Both minimum and maximum interpass temperatures may be specified - minimum to maintain preheating effectiveness, and maximum to prevent excessive grain growth or, in the case of stainless steel, sensitization.
Preheat and Interpass Temperature for A514 Steel
A514 (T-1) is a quenched and tempered (Q&T) high-strength structural steel, highly susceptible to hydrogen-induced cracking. Per CSA W59:24 Table 5.3, both a minimum preheat and a maximum interpass temperature are specified - and these serve opposite purposes.
Minimum preheat temperatures for A514 (per CSA W59:24 Table 5.3):
| Plate Thickness | Minimum Preheat |
|---|---|
| Up to 19 mm | 10 degrees C |
| 19 mm to 38 mm | 50 degrees C |
| 38 mm to 64 mm | 105 degrees C |
Maximum interpass temperature: 205 degrees C (400 degrees F) for all thicknesses.
This maximum limit is critical and often confused with the minimum preheat. Over-heating between passes tempers the quenched and tempered martensite microstructure, destroying the yield strength that A514 was engineered to provide. The 205 degrees C maximum interpass temperature prevents this over-tempering.
Minimum Preheat (by thickness)
Purpose: Slow cooling rate, allow hydrogen to escape, reduce martensite formation
Up to 19 mm: 10 degrees C
19-38 mm: 50 degrees C
38-64 mm: 105 degrees C
Maximum Interpass Temp: 205 degrees C
Purpose: Prevent over-tempering the Q&T martensite
Risk if exceeded: Loss of yield strength - defeats the purpose of using A514
Applies to: All thicknesses
Critical Distinction for A514
200 degrees C is NOT the minimum preheat - it is approximately the maximum interpass temperature. Confusing these values is a common exam error. The minimum preheat ranges from 10 degrees C (thin) to 105 degrees C (thick). Do not confuse preheat with PWHT either: preheating occurs before and during welding; PWHT occurs after welding to relieve residual stresses.
Preheating reduces cooling rate and prevents cracking. For A514 steel, minimum preheat ranges from 10 degrees C (thin) to 105 degrees C (64 mm) per CSA W59:24 Table 5.3. The maximum interpass temperature is 205 degrees C - exceeding it over-tempers the Q&T microstructure and destroys yield strength.