About Post Weld Heat Treatment,Top 4 Things You Should Know.

       

When you are unsure whether post HT is required, you must consider alloy system or previous heat treatment of the base metal. For example, properties of quenched or tempered alloy steals can be adversely affected by post heating. This is particularly true if the temperature exceeds the tempering temperature of the base metal.

Most applications do not require post heating. However, this may be a code requirement through the National Board Inspection Code (NBIC). NBIC requires post heating treatment to be between 500 to 550°F (260 to 290°C) for a minimum of 2 hour. In general, post heating is intended to reduce potential hydrogen cracking problems due to sensitive metal bases and microstructure. PT is only required when there is a potential issue with hydrogen cracking. ASME Section III also requires post heating along with NBIC.

After welding, it is very important that the steel does not cool to room temperature. Rather, it should be heated from the interpass temperature to the PH temperature. Common post heating temperature is around 450 degrees Fahrenheit. It is very important that the welded structure does not drop to room temperature as it will cause potential problems of cracking.

Stress relieving of heated and welded structures is meant to reduce the stress as a result of the manufacturing process. In many cases, there are residual stresses due to welding. Therefore, it is very important to apply proper stress relieve techniques to minimize this potential problem. When proper post heating treatment is not applied, it may cause the structure to crack immediately or later on which can be dangerous if the structure being welded is a supporting piece of a larger product.