Welding estimation and testing methods of metal materials and their performance influencing factors

Date：2022-02-16 18:17:03Views：957

Weldability of metal materials refers to the ability of metal materials to obtain excellent welded joints under certain welding processes, including welding methods, welding materials, welding specifications and welding structure forms. In order to help you have an in-depth understanding, this paper will summarize the relevant knowledge of metal material welding. If you are interested in what this article will cover, read on.

Estimation and testing method of weldability of metal materials

1. Indirect evaluation method of process weldability

Since the influence of carbon is the most obvious, and the influence of other elements can be converted into the influence of synthetic carbon, the excellent weldability is evaluated by carbon equivalent.

Carbon equivalent calculation formula of carbon steel and low alloy structural steel:

CE under general welding technical conditions, the welded joint will not produce cracks, but preheating should be considered for thick and large parts or welding at low temperature;

When CE is 0.4 ~ 0.6%, the plasticity of steel decreases, the hardening tendency increases gradually, and the weldability is poor. The workpiece shall be preheated properly before welding, and pay attention to slow cooling after welding to prevent cracks;

When CE > 0.6%, the plasticity of steel becomes worse. The hardening tendency and cold cracking tendency are large, and the weldability is worse. The workpiece must be preheated to a higher temperature, technical measures to reduce welding stress and prevent cracking shall be taken, and appropriate heat treatment shall be carried out after welding.

The larger the carbon equivalent value obtained from the calculation results, the greater the hardening tendency of the welded steel, and the cold crack is easy to occur in the heat affected zone. Therefore, when CE > 0.5%, the steel is easy to harden, and the crack must be prevented by preheating during welding. With the increase of plate thickness and CE, the preheating temperature should also increase accordingly.

2. Direct evaluation method of process weldability

According to the welding crack test method, the cracks in the welded joint can be divided into hot crack, cold crack, reheat crack, stress corrosion, layered tear, etc.

(1) T-joint welding crack test method, which is mainly used to evaluate the thermal crack sensitivity of fillet welds of carbon steel and low alloy steel, and can also be used to determine the influence of welding rod and welding parameters on the thermal crack sensitivity.

(2) Test method for butt welding crack of pressing plate, which is mainly used to evaluate the thermal crack sensitivity of carbon steel, low alloy steel, austenitic stainless steel electrode and weld. By installing the test piece in the FISCO test device, adjusting the groove gap has a great impact on the generation of cracks. With the increase of gap, the greater the crack sensitivity.

(3) Rigid butt crack test method. This method is mainly used to measure the hot crack and cold crack in the weld area, and can also measure the cold crack in the heat affected area. The surrounding of the test piece shall be firmly welded on the rigid base plate with tack weld. During the test, the test weld shall be welded according to the actual construction welding parameters. It is mainly used for electrode arc welding. The test piece shall be placed at room temperature for 24 hours after welding, and the weld surface shall be inspected first, Then cut off the sample grinding plate and check whether there is crack. Generally, crack and non crack are the evaluation criteria, and two test pieces are welded under each condition.

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. Details are as follows:

1. Material factors

Materials include base metal and welding materials. Under the same welding conditions, the main factors determining the weldability of base metal are its own physical properties and chemical composition.

Physical properties: such as melting point, thermal conductivity, coefficient of linear expansion, density, heat capacity and other factors of the metal, all have an impact on the thermal cycle, melting, crystallization, phase transformation and other processes, thus affecting the weldability. Stainless steel and other materials with low thermal conductivity have large temperature gradient, high residual stress and large deformation during welding,. Moreover, due to the long residence time at high temperature, the grains in the heat affected zone grow up, which is unfavorable to the performance of the joint. The linear expansion coefficient of austenitic stainless steel is large, and the deformation and stress of the joint are serious.

In terms of chemical composition, the most influential element is carbon, that is, the amount of carbon in the metal determines its weldability. Most of the other alloying elements in steel are also not conducive to welding, but their influence is generally much smaller than that of carbon. With the increase of carbon content in steel, the hardening tendency increases and the plasticity decreases, which is easy to produce welding cracks. Generally, the sensitivity of metal materials to cracks during welding and the change of mechanical properties of welded joint area are regarded as the main indexes to evaluate the weldability of materials. Therefore, the higher the carbon content, the worse the weldability. Low carbon steel and low alloy steel with carbon content less than 0.25% have excellent plasticity and impact toughness, and the plasticity and impact toughness of welded joints after welding are also very good. Preheating and post weld heat treatment are not required during welding, and the welding process is easy to control, so it has good weldability.

In addition, the smelting and rolling state, heat treatment state and microstructure state of steel affect the weldability to varying degrees. The weldability of steel can be improved by refining or refining grain and controlled rolling process.

Welding materials directly participate in a series of chemical and metallurgical reactions in the welding process, which determines the composition, microstructure, properties and defect formation of weld metal. If the welding material is selected improperly and does not match with the base metal, it will not only fail to obtain the joint that meets the service requirements, but also introduce the generation of defects such as cracks and the change of microstructure and properties. Therefore, the correct selection of welding materials is an important factor to ensure the acquisition of high-quality welded joints.

2. Process factors

Process factors include welding method, welding process parameters, welding sequence, preheating, post heat and post weld heat treatment. Welding methods have a great influence on weldability, mainly in two aspects: heat source characteristics and protection conditions.

Different welding methods have different heat sources in terms of power, energy density and maximum heating temperature. When metals are welded under different heat sources, they will show different welding properties. For example, the power of electroslag welding is large, but the energy density is very low, the maximum heating temperature is not high, the heating is slow during welding, and the high temperature residence time is long, resulting in coarse grains in the heat affected zone and significantly reduced impact toughness, which can be improved only after normalizing. On the contrary, electron beam welding, laser welding and other methods have low power, but high energy density and rapid heating. The high temperature residence time is short, the heat affected zone is very narrow, and there is no danger of grain growth.

By adjusting the welding process parameters and taking other process measures such as preheating, post heating, multi-layer welding and controlling the interlayer temperature, the welding thermal cycle can be adjusted and controlled, so as to change the weldability of metal. If measures such as preheating before welding or heat treatment after welding are taken, it is entirely possible to obtain a welded joint without crack defects and meeting the service performance requirements.

3. Structural factors

It mainly refers to the design form of welding structure and welded joint, such as the influence of structural shape, size, thickness, joint groove form, weld arrangement and section shape on weldability. Its influence is mainly reflected in heat transfer and force state. The heat transfer rate of the groove is different from that of the crystal pool, which has the same effect on the heat transfer rate or the growth direction of the crystal pool. The switch, plate thickness and weld arrangement of the structure determine the stiffness and restraint of the joint and affect the stress state of the joint. Poor crystal morphology, serious stress concentration and excessive welding stress are the basic conditions for the formation of welding cracks. In the design, reducing the stiffness of joints, reducing cross welds and reducing various factors causing stress concentration are important measures to improve weldability.

4. Service conditions

It refers to the working temperature, load conditions and working medium of the welded structure during service. These working environment and operating conditions require the welded structure to have corresponding performance. For example, the welded structure working at low temperature must have brittle fracture resistance; The structure working at high temperature should have creep resistance; The structure working under alternating load has good fatigue resistance; Welding vessels working in acid, alkali or salt media shall have high corrosion resistance, etc. In short, the more severe the service conditions are, the higher the quality requirements for welded joints are, and the more difficult it is to ensure the weldability of materials.