CLADDING TECHNOLOGY SHANXI CO., LTDAmbient Temperature Lower Limit and Welding Permission Criteria for Cladding Operations
Overview and Metallurgical Basis
The ambient temperature at which welding is performed has a profound influence on the metallurgical quality of the weld, particularly for cladding operations involving dissimilar material combinations such as austenitic stainless steel on carbon steel, nickel-based alloys on low-alloy steel, and titanium on steel. The ambient temperature affects the cooling rate of the weld, the heat input distribution, the residual stress state, and the susceptibility of the weld to hydrogen-induced cracking (HIC), sulfide stress corrosion (SSC), and cold cracking.
The widely adopted criterion that ambient temperature must be at least 5 °C for welding to proceed without additional measures is based on extensive field experience and metallurgical understanding. Below this threshold, the cooling rate of the weld increases significantly, the heat input is distributed over a smaller volume of the base metal, and the susceptibility to cold cracking increases. For carbon steel and low-alloy steel substrates, the increased cooling rate can result in the formation of martensitic microstructures in the heat-affected zone (HAZ), which are susceptible to HIC and SSC under the conditions encountered in sour service.
Metallurgical Mechanisms
Cooling Rate and Microstructural Evolution
The ambient temperature directly influences the cooling rate of the weld. A lower ambient temperature increases the temperature gradient between the weld pool and the surrounding base metal, accelerating the cooling rate. For carbon steel substrates, a cooling rate above 10 °C/s in the 800–500 °C range can result in the formation of martensite and bainite in the HAZ, which are hard and brittle microstructures susceptible to cold cracking. For austenitic stainless steel cladding layers, a rapid cooling rate can result in the formation of delta ferrite in excess of the specified limit, which can compromise the corrosion resistance and mechanical properties of the cladding layer.
Hydrogen Embrittlement
The ambient temperature also affects the diffusion and entrapment of hydrogen in the weld metal. At lower temperatures, the diffusion coefficient of hydrogen in the weld metal decreases, resulting in a higher concentration of trapped hydrogen. This increases the susceptibility of the weld to hydrogen-induced cracking, particularly in high-strength weld metals and in welds with high residual stress. The 5 °C threshold is partly based on the observation that below this temperature, the hydrogen entrapment risk increases significantly for most welding consumables.
Preheat Temperature Requirements
The ambient temperature criterion is closely related to the preheat temperature requirement. For carbon steel and low-alloy steel substrates, the preheat temperature is specified to reduce the cooling rate and to promote the diffusion of hydrogen out of the weld. The preheat temperature is typically specified as a function of the carbon equivalent (CE) of the base material, the thickness of the base metal, and the ambient temperature. At lower ambient temperatures, the required preheat temperature increases to maintain the same cooling rate.
| Base Material | CE Range | Ambient ≥5 °C | Ambient <5 °C |
|---|---|---|---|
| Carbon steel (S355) | <0.40 | Preheat 50 °C | Preheat 100 °C + enclosure |
| Low-alloy steel (P355GH) | 0.40–0.60 | Preheat 100 °C | Preheat 150 °C + enclosure |
| Low-alloy steel (12Cr1MoV) | >0.60 | Preheat 150 °C | Preheat 200 °C + enclosure |
| Austenitic SS (316L) | N/A | No preheat | No preheat (cladding only) |
Implementation in WPS and ITP
The ambient temperature criterion must be explicitly stated in the Welding Procedure Specification (WPS) and the Inspection and Test Plan (ITP). The WPS must specify:
- The minimum ambient temperature for welding without additional measures (typically 5 °C).
- The measures to be taken when the ambient temperature is below the minimum threshold, including the required preheat temperature, the use of a heated enclosure, and the monitoring procedure.
- The method of measuring the ambient temperature, including the location of the temperature sensor (typically at the welding position, 1 m from the workpiece) and the frequency of measurement (typically every 30 minutes).
- The acceptance and rejection criteria for the ambient temperature measurement.
The ITP must specify the inspection points at which the ambient temperature is verified, including the hold point before welding starts and the witness point during welding. The quality inspector must verify the ambient temperature at each hold point and record the measurement in the welding log.
Engineering Practice Considerations
In practice, the implementation of the ambient temperature criterion faces several challenges in northern regions where winter temperatures can drop well below 5 °C for extended periods. The use of heated enclosures is a common solution, but the enclosure must be designed to maintain a uniform temperature distribution and to prevent moisture condensation, which can introduce hydrogen into the weld. The enclosure must also be ventilated to prevent the accumulation of welding fumes and to provide adequate visibility for the welder.
For outdoor welding operations in cold environments, the use of portable preheaters and heated welding positions is common. The preheater must be designed to heat the base metal to the required preheat temperature without overheating the surrounding area, which can cause thermal distortion. The temperature of the base metal must be measured at the welding position using a contact thermometer or an infrared pyrometer, and the measurement must be recorded in the welding log.
For cladding operations involving nickel-based alloys such as Inconel 625 or Hastelloy C276, the ambient temperature criterion is less critical than for carbon steel and low-alloy steel substrates, as the austenitic microstructure of the cladding layer is not susceptible to cold cracking. However, the ambient temperature still affects the cooling rate and the dilution ratio, and the WPS must specify the appropriate measures for cold-weather welding.
Study Insights
The ambient temperature criterion is one of the most frequently violated quality requirements in winter welding operations, particularly in northern regions of China and other cold climates. The key insight is that the 5 °C threshold is not an arbitrary number but is based on metallurgical principles that govern the cooling rate, hydrogen entrapment, and cold cracking susceptibility. The criterion must be enforced with the same rigor as other WPS parameters, and the quality inspector must have the authority to stop welding when the ambient temperature is below the specified limit. The long-term investment in heated enclosures and preheating equipment is justified by the reduction in rework costs and the improvement in weld quality.