How to Control the Quality of Brewery Equipment – Discussion of Issues Related to Post-Weld Heat Treatment in Pressure Vessels
Heat treatment has always been a relatively weak link in the design and manufacturing of pressure vessels. It is a traditional and effective method for improving and restoring metal properties. Pressure vessels, such as brewery equipment (including brewhouses, fermenters, and bright beer tanks), involve four types of heat treatment: post-weld heat treatment (stress relief heat treatment), heat treatment for improving material properties, heat treatment for restoring material properties, and post-weld hydrogen elimination treatment. In this discussion, we will focus on the issues related to post-weld heat treatment, which is widely used in the design of pressure vessels.
1. Purpose of Post-Weld Heat Treatment (Stress Relief Heat Treatment)
The purpose of post-weld heat treatment is as follows:
(1) Relieve welding-induced stress.
(2) Stabilize the shape and size of the structure and reduce distortion.
(3) Improve the performance of the base metal and weld zone by:
– Enhancing the plasticity of the weld metal.
– Reducing the hardness of the heat-affected zone.
– Improving fracture toughness.
– Enhancing fatigue strength.
– Restoring or increasing the yield strength reduced during cold forming.
(4) Improve the ability to resist stress corrosion.
(5) Eliminate harmful gases, especially hydrogen, in the weld metal to prevent the occurrence of delayed cracks.
2. Does Stainless Steel Pressure Vessels Require Post-Weld Heat Treatment?
Post-weld heat treatment is widely used in pressure vessels made of carbon steel and low alloy steel with a body-centered cubic crystal structure. However, stainless steel has a face-centered cubic crystal structure, which exhibits good toughness and strain strengthening properties. Since stainless steel pressure vessels are often used for anti-corrosion purposes and have specific temperature requirements, their wall thickness is usually not high. Therefore, post-weld heat treatment is not necessary for stainless steel pressure vessels unless there are abnormal operating conditions or specific research indicates otherwise. It is important to consider corrosion, material instability, fatigue, impact load, and other factors during the design phase.
3. Heat Treatment of Explosive Stainless Steel Clad Steel Plate Vessels
Explosive stainless steel clad steel plate is widely used in the pressure vessel industry due to its excellent corrosion resistance, mechanical strength, and cost-effectiveness. However, the heat treatment of this material should be considered by pressure vessel designers. The process of explosive processing of metal composite panels involves applying energy to the metal surface, resulting in a bonded interface between the composite metal and the base metal. Heat treatment considerations should not be overlooked when using explosive stainless steel clad steel plates in pressure vessels.
4. Can Alternative Methods Replace Overall Heat Treatment?
Due to manufacturing plant constraints and economic interests, alternative methods have been explored to replace overall heat treatment for pressure vessels. These methods, such as local heat treatment, hammering method, explosion method, and vibration method, have their benefits but cannot fully replace overall heat treatment according to current standards and regulations. Although some methods, like the hammering method and the explosion method, show promise in eliminating welding residual stress, they have not been widely adopted due to a lack of quantitative indicators, stricter operating procedures, and insufficient verification. Overall heat treatment remains the widely accepted method for eliminating welding residual stress in the pressure vessel industry, despite its drawbacks of high energy consumption and long cycle.