What factors determine the weldability of metal materials?

Date：2021-09-18 15:03:05Views：1930

Weldability refers to the ability of materials to be welded into components specified by design requirements and meet predetermined service requirements under specified welding conditions. For the metal with good weldability, the welded joint is not easy to produce cracks, pores and slag inclusion defects, and has high mechanical properties. The weldability of metal materials refers to the difficulty of obtaining high-quality welded joints under certain welding methods, welding materials, process parameters and structural types. The main factor of weldability of steel is chemical composition. Among various elements, the influence of carbon is the most obvious, and the influence of other elements can be converted into the influence of carbon. Therefore, the weldability of welded steel can be estimated by carbon equivalent method. Sulfur and phosphorus also have a great impact on the welding performance of steel. In all kinds of qualified steel, sulfur and phosphorus should be strictly limited.

The weldability of metal materials includes two aspects: one is the bonding performance, that is, the sensitivity of a certain metal to form welding defects under certain welding process conditions; the other is the service performance, that is, the applicability of a certain metal welded joint to the application requirements under certain welding process conditions. There are many factors affecting the weldability of metal materials, mainly including metal materials, structural design, process measures and service environment. Weldability is a comprehensive performance that depends on the chemical composition of base metal and weld metal, welding structure, design of welded joint, welding method, welding process, etc.

Material factors

Material factors refer to wood itself and welding materials; Including the chemical composition, smelting and rolling state, heat treatment, microstructure and mechanical properties of the material.

Welding materials such as welding rod for arc welding, welding wire and flux for submerged arc welding, welding wire and shielding gas for lifting ladle arc welding, etc. In the welding process, wood and welding materials directly participate in the metallurgical reaction in the molten pool or fusion zone, which has an important impact on weldability and welding quality. When the base metal or welding material is not selected properly, the weld composition will be unqualified, the mechanical properties and other properties will be reduced, and even welding defects such as cracks, pores and slag inclusion will appear, that is, the welding process will become worse, so it must be selected correctly.

In terms of base metal, chemical composition has the greatest influence. If the steel only relies on alloy elements to realize solid solution strengthening, it is generally easy to make the weld metal, heat affected zone and base metal have good matching properties in the welding process. If the steel is a complex alloy system and strengthened by heat treatment and deformation processing, it is not easy to obtain the weld metal completely matching the base metal, or even the whole welded joint. For steel, the elements that have great influence on weldability include C, P, h, s, O and N. Mn, Si, Cr, Ni, Mo, Ti, V, Nb, Cu and B in alloy elements may increase the hardening tendency and crack sensitivity of welded joints to varying degrees. Generally speaking, the weldability of steel will deteriorate with the increase of carbon content and alloy element content.

The weldability is also affected to varying degrees in smelting method, rolling process and heat treatment state. At present, CF steel (crack resistant steel), Z-direction steel and TMCP Steel (controlled rolling steel) all improve the weldability of materials by refining and refining, controlling rolling process and other means.

Structural factors

Structural factors include the form, stiffness and stress state of welded structure and welded joint,

These will affect the mechanical properties of the joint and produce welding defects.

For welded structures with volume and weight requirements, materials with higher specific strength (such as light alloy materials) shall be selected in the design to reduce volume and weight. For welded structures without special requirements for volume and weight, the selection of materials with high strength grade also has its technical and economic significance, which can reduce the self weight of the structure, save base metal and welding materials, avoid the difficulties in hoisting and transportation of large structures, and improve the bearing capacity.

The structural design of welded joints will affect the stress state, thus affecting the weldability. In the design, the stress at the joint should be in a small state and can shrink freely, which is conducive to reducing stress concentration and preventing welding cracks. Try to avoid stress concentration caused by notch, sudden change of section, excessive height and cross weld at the joint. It is also necessary to avoid multi-directional stress caused by unnecessary increase of base metal thickness or weld volume.

The characteristics of welding heat source, power density, linear energy and other process parameters will directly determine the temperature field and thermal cycle of the joint, which has an obvious impact on the range of weld and heat affected zone, microstructure change and defect sensitivity. For example, preheating before welding and slow cooling after welding can reduce the cooling rate of the joint, the hardening tendency and cold crack sensitivity of the joint. Choosing a reasonable welding sequence can improve the constraint degree and profitability of the structure. Argon arc welding and other welding methods can protect the welding area tightly, reduce the burning loss of alloy elements, and obtain satisfactory joint performance.

Process factors

The process factors include the welding method adopted during construction, welding process specification (such as welding heat input, preheating, welding sequence, etc.), post weld heat treatment, etc.

For the same base metal, different welding methods and process measures show great differences in weldability. For example, aluminum and its alloys are difficult to weld with gas welding, but good results can be achieved with argon arc welding; Titanium alloy is very sensitive to, O, h and n. It is impossible to weld well with gas welding and electrode arc welding, but it is easier to weld with argon arc welding or electron beam welding. Therefore, developing new welding methods and processes is an important way to improve the weldability of processes.

The influence of welding method on weldability is firstly reflected in the energy density, temperature and heat input of welding heat source; Secondly, it is manifested in the ways of protecting the molten pool and near the joint, such as slag protection, gas protection, slag gas combined protection, welding in vacuum, etc. For high strength steel with overheating sensitivity, narrow gap gas shielded welding, pulse arc welding and plasma arc welding can be selected from the perspective of preventing overheating, which is conducive to improving its weldability.

The most common process measures are preheating before welding, slow cooling and post welding heat treatment. These process measures are effective measures to prevent hardening and embrittlement of heat affected zone, reduce welding stress and avoid hydrogen cooling cracks. Reasonable arrangement of welding sequence can also reduce stress and deformation. In principle, the welded parts shall be in a state of unrestricted and free expansion and contraction during the whole welding process. Post weld heat treatment can eliminate residual stress and prevent delayed crack by allowing hydrogen to escape. In addition, gas cutting, cold working, assembly and other processes of metal materials before welding shall comply with the characteristics of materials to avoid local hardening, embrittlement, stress concentration and welding cracks.

environmental factor

Environmental factors refer to the working temperature, load conditions (such as load type, action mode, speed, etc.) and working medium of the welded structure.

The service environment of welded structures is diverse. Welded structures working at high temperature require materials with high enough high temperature strength, good chemical stability and structural stability, high creep strength and so on; The welding structure working at room temperature requires the material to have good mechanical properties in the natural environment; When the working temperature is low or subjected to impact load, special attention shall be paid to the performance of the material at the lowest ambient temperature, especially the impact toughness, so as to prevent low-temperature brittle failure. For the components bearing East load, the materials are required to have good dynamic fracture toughness and vibration absorption. When the working medium is corroded, the welding area is required to have corrosion resistance. For welded structures working in nuclear radiation environment, due to the action of neutron radiation, the yield point of materials will increase, the plasticity will decrease, the brittle transition temperature will increase, and the toughness will decrease, resulting in irradiation brittleness of materials.

This article introduces the weldability of metal materials. Through this article, I hope it can be helpful to you. If you like this article, you might as well continue to pay attention to our website, and we will bring more wonderful content later. If you have any needs related to the inspection and testing of electronic products, please call Chuangxin testing, and we will serve you wholeheartedly.