The ability to be welded is determined by several factors, including:
- Welding Metallurgy
- Welding Chemistry
- Connection Surface Condition
- Geometry (shape) Connection
Welding Metallurgy, is a change that occurs in a metal that experiences various kinds of mechanical and thermal effects in a welding process. Welding Metallurgy depends on the arrangement of atoms and how the arrangement of the atoms is influenced by style and heat. The type of arrangement of metal atoms causes differences in their mechanical properties. Thus we can see the relationship between metallurgical treatment of metals, namely preheating, final heating, stress relief etc. with the resulting mechanical capabilities.
Welding Chemistry is a chemical relationship between base metals, filler metals, and other chemicals present in welding process. The ability of the base metal and filler metal to fuse without causing a bad chemical effect is of great importance in relation to weld ability.
Joint Surface Condition (connection surface conditions) and Joint Geometry (connection shape) are the final factors affect the ability to be welded (weld ability). The condition of the joint surface includes the effects of roughness and cleanliness joint surface. The shape / geometry of the joint also affects weldability. The amount of voltage also affects weldability. Even though the weld ability problem can be solved by the engineers, the welding inspector must keep in mind that the weld ability problem still exists. Repetitive welding defects or not due to welder’s fault must be noted and repaired. By knowing welding metallurgy and welding chemistry, the Welding Inspector is better able to anticipate weldability problems by knowing the initial signs.
Metallurgy is the science of metal structure and its relationship with the metal’s capabilities. Topics related to welding are:
- Solids and liquids
- Melting and freezing
- Heat expansion
- Heat treatment
- Blends and alloys
Welding seen under a microscope.
In solid metals the atoms tend to arrange themselves in lines, rows and layers form a 3-dimensional crystal structure. Common metal crystal structures are BCC (body centered cubic), FCC (face centered cubic), and HCP (hexagonal close packed). Some metals such as iron have several different structures depending on the the temperature.
Steel has several phases, namely austenite, ferrite, perlite, bainite, martensite. At a temperature of 1333 F with a content of 0.3% C in the form of ferrite and pearlite. Above 1333 F the phase is a mixture of austenite and ferrite, and above 1550 F is austenite. By varying the cooling rate of the austenite we can adjust the phase of the steel. Fast cooling causes the steel to be martensite, slow cooling to ferrite and pearlite form, while intermediate cooling is bainite.
Martensitic steel requires a heat treatment in the form of “tempering”. Tempering is done by heating the steel between 100 – 1300 F to soften it, at low temperature there is no visible change in phase, but the strength and hardness decrease, while the toughness and ductility increase.
The area of the parent metal that is affected by the heat of welding is called the Heat Affected Zone (HAZ). In this HAZ area there is a tendency for high hardness and low ductility. To reduce this tendency, a heat treatment called preheating is carried out. By heating the parent metal before welding at a temperature of 150 – 700 F (65 – 370 C), the cooling rate will decrease. By slowing cooling the formation of the martensite structure is avoided, a softer but more ductile bainite or ferrite – pearlite structure is formed, thereby reducing the tendency to break in the weld and the HAZ area.
Another factor that affects the cooling speed is the heat input, the more heat input the cooling speed drops. This is overcome by using a small electrode diameter, a lower current and a higher welding speed (traveling speed), in other words a smaller heat input.
Other Metallurgical Factors
All welds are designed to withstand plastic deformation under load (up to the yield point), except for fatigue. The welded material is designed for low loads to avoid fatigue.
- Surface Shape.
Another important factor for avoiding fatigue is surface shape. The sharp shape creates a “stress increase”. Broken with a sharp shape can increase stress up to 10 times. Under cut can increase stress 3 – 5 times, and curly welds increase stress 2-3 times.
- Inner structure.
When the metallic liquid solidifies, it becomes large ingots that have a pouring structure. The ingots are formed into rolling plates which cause the metal grains to become flat and have poor mechanical properties (hard, brittle). Mechanical strength is very good at direction rolling and poor in the transverse direction of rolling.