The quality of the welded joint is influenced not only by the contractor's experience, consumables and equipment, but also by the physical and chemical characteristics of the metal being welded.
The weldability of precision alloys and steels is divided into technological and physical weldability. The first determines how the metal will react to welding and whether it will make it possible to create an inseparable joint that meets performance requirements. The second characterizes the ability of the material to form interatomic bonds at the interface between the base and the welded metal.
How is the weldability of steels and precision alloys assessed
The weldability of materials is a complex and multifactorial property that depends on the ability of a metal to react chemically, the spatial arrangement of its atoms and ions, and the amount of carbon and other impurities.
The main evaluation criteria are:
- sensitivity to thermal effects — indicates grain growth, as well as changes in the metal structure in the seam and the heat-affected zone of welding, affects the strength and plasticity of the finished structure or part;
- resistance to hot cracks — indicates the ability precision steels and alloys to withstand high temperatures without forming internal defects;
- the tendency to form oxide films depends on the chemical activity of the metal, oxidizability in places of heating complicates the welding process;
- resistance to cold cracks — characterizes the possible formation of defects in welded joints when welds are cooled;
- tendency to form pores — can reduce the strength of an inseparable joint and prevent the creation of a sealed seam.
In addition to the main indicators, they also take into account the level of natural stresses and internal deformations of the materials and products being welded, and the quality of weld formation.
What alloying additives affect weldability
Carbon plays a decisive role in determining the properties of steel and its ability to process it, including welding. Alloys with low and medium C content are well welded. Metals containing more than 0.35% of this chemical element require certain welding technologies. The higher the carbon content, the more likely it is that cracks and other hardening structures will form in the seam zones and pores in the suture body.
There are other alloying additives that affect the weldability of materials:
- nickel — has a positive effect on plasticity, strength and corrosion resistance, and also improves weldability, however, when welding materials with a high nickel content, additional protection is required to prevent this chemical element from burning out when it interacts with oxygen;
- chromium — although it increases the corrosion resistance and strength characteristics of metals, at a content of more than 0.3% it can cause rust at the welded joint;
- manganese — if present above 1.8%, it makes steel stronger and harder, which leads to cracks when heated; if the manganese content is more than 11%, then Mn electrodes should be used for welding;
- titanium — increases strength and impact strength, improves the weldability of materials, can mitigate hardening phenomena and prevent the formation of chromium carbides, which is especially important when working with chromium-containing metals;
- silicon — at a concentration of 0.8% to 1.5%, it can make welding difficult, creating fluid oxides and contributing to the slagging of the seam;
- sulphur et phosphorus — when one of them contains more than 0.05%, they cause the formation of iron sulfide, which contributes to the appearance of cracks when the welded joint is cooled.
It is important to take into account the content of these impurities and take appropriate measures when welding steel in order to ensure a high-quality and reliable connection.
Weldability groups for precision and other alloys
The main method for determining the degree of weldability of materials includes the manufacture of samples on which rollers are welded. After mechanical, chemical and heat treatment, the obtained samples are subjected to ultrasonic and magnetic flaw detection, mechanical tests for bending, tensile, hardness and impact strength, and other types of studies. The results obtained fall into one of four groups:
- Good weldability — the material is considered to be well welded if cracks do not form during the welding process. Examples of steels: 20880, 20895, 27KH, 08KP, St20.
- Satisfactory weldability — cracks occur as a result of cooling a welded joint in an aqueous medium, but are absent when cooled in air. Examples: 12HN3A, 20HGSA, 30HM, ST35.
- Limited weldability — this means that in order to prevent the formation of cracks during welding, it is necessary to preheat the material to a temperature of 100—150°C and then cool it in the air. Some steel grades: 12X18N9, 20X2N4MA, 30HGSA, St45.
- Poor weldability — steel requires special welding techniques and significantly higher preheating (up to 300°C and above) to avoid cracking, and heat treatment after creating a welded joint. Examples of alloys: 50HGA, 60S2A, U8A, St70.
St. Petersburg Precision Alloy Plant produces steel with different levels of weldability. If you want to learn more about the materials presented, as well as find the most suitable alloy for your project, call us or leave requests on the site. Our engineers will contact you and answer all your questions in detail.
