Weld and heat treatment

1. Classification of welds

The welds of each part of the pressure vessel are divided into four categories: A, B, C, and D. Class A welds refer to the longitudinal welds of the equipment; Class B welds refer to the circumferential welds; Class C welds refer to the welds between the flat welded flanges and the nozzles; Class D welds refer to the welds between the equipment nozzles and the barrel. 2. Heat treatment

(I. Overview

1. The concept of heat treatment

The process of heating the metal to a given temperature and maintaining it for a period of time, and then selecting the speed and method to cool it to obtain the required microstructure and properties, is called heat treatment. The heat treatment in construction generally refers to the heat treatment of the welded joint (heat-affected zone). The heat treatment process of the welded joint (heat-affected zone) is the process of uniformly heating the welded joint to a certain temperature, keeping it warm, and then cooling.

2. The meaning of heat treatment

The heat treatment of the welded joint can prevent brittle damage, delayed cracking, stress corrosion and hydrogen corrosion of the welded part. After correct heat treatment, the welding residual stress can be relaxed, the hardened zone can be softened, the structure can be improved, the hydrogen content can be reduced, and the corrosion resistance, impact toughness, creep limit, etc. can be improved. However, improper heat treatment of welded joints will reduce the performance of the joints.

(2) Preheating before welding

Preheating is a major technological measure during welding, especially for welding thick workpieces. Preheating the weldment before welding can prevent or reduce stress. For welding some important structures, such as high-pressure thick-walled vessels or weldments with poor plasticity and strong quenching tendency, preheating is generally required before welding to prevent cracks during welding. The function of preheating is to reduce the temperature difference between the weld metal and the base metal, that is, to increase the initial temperature of the welded joint, thereby reducing the shrinkage stress, reducing the cooling rate of the weld, controlling the structural transformation of the steel, and avoiding the formation of brittle horses in the heat-affected zone. Stenite reduces local hardening and improves the quality of welds. Because preheating is beneficial to exhaust and slag removal, it can reduce defects such as pores and slag inclusion.

Whether the weldment needs to be preheated and what the preheating temperature should be based on the chemical composition and thickness of the steel plate, the structural rigidity of the container, the welding form, the welding method and the welding material, and the environmental temperature should be considered comprehensively.