During fusion welding, not only a series of changes from melting to solid phase change occur in the weld under the action of welding heat source, but also the structure and properties of the base metal on both sides of the weld that is not melted will change due to the influence of welding heat transfer.
In addition, there is a transition zone from the base metal to the weld, which is different from the weld and the base metal, which will have a great impact on the performance of the welded joint.
1 – Welding seam
2 – Fusion Zone
3 – Heat affected zone
4 – Base metal
1. Structure and performance of fusion zone
The fusion zone refers to the area where the weld joint transits to the heat affected zone in the welded joint.
The area is very narrow and difficult to distinguish even under a microscope.
Superheated structure is one of the internal defects of steel.
It is a metal structure characterized by large grain size formed by heating temperature exceeding AC3 for a long time or staying at high temperature.
The temperature of the fusion zone is between the solid phase line and the liquid phase line in the state diagram of the iron carbon alloy.
The metal in this area is in a partially melted state (semi molten zone), with very coarse grains.
After cooling, the structure is a coarse overheated structure, with poor plasticity and toughness.
Because the fusion zone has obvious chemical heterogeneity and structural heterogeneity, it is often the source of cracks or local brittle failure of welded joints, and the area with the worst neutral energy of welded joints.
2. Welding thermal cycle
Under the action of welding heat source, the process that the temperature of a point on the weldment changes with time is called welding thermal cycle.
Welding thermal cycle refers to a specific point on the weldment, when the heat source is close to the point.
The temperature at this point increases until it reaches the maximum value.
With the heat source leaving, the temperature gradually decreases to room temperature.
This process can be represented by a curve.
Welding thermal cycle curve
Tm – maximum temperature of heating
TA – phase transition temperature
tA – residence time above phase transition temperature
3. Structure and properties of welding heat affected zone
Welding heat affected zone refers to the area where the metallographic structure and mechanical properties of the base metal change due to the influence of heat (but not melting) during the welding process.
The structure and properties of HAZ basically reflect the properties and quality of welded joints.
For low-carbon steel and low-alloy high-strength steel with few alloying elements, the welding heat affected zone can be divided into overheated zone, normalizing zone, incomplete recrystallization zone and recrystallization zone.
Welding heat affected zone of non quenchable steel
1. Fusion zone
2. Overheated area
3. Normalizing area
4. Incomplete recrystallization zone
5. Recrystallization zone
6. Base metal
Overheated structure: widmanstatten structure
In the superheated zone of zone welding heat effect, because the austenite grain is very coarse, this coarse austenite will form a special superheated structure under a relatively fast cooling rate.
Its structural characteristics are that many parallel ferrite (cementite) needles will be formed in a coarse austenite grain, and the remaining austenite between the ferrite needles will finally be transformed into pearlite.
This overheated structure is called ferrite (cementite) widmanstatten structure.
In short, when the austenite grain is relatively coarse and the cooling rate is appropriate, the pre eutectoid phase in the steel is mixed with flake pearlite in the form of needle flake.
The widmanstatten structure not only has large grain size, but also the flexibility of the metal is rapidly reduced due to the fragile surface formed by a large number of ferrite needles, which is a main reason for the embrittlement of the welded joint of the steel that is not easy to quench.
The width of heat affected zone is related to welding method, welding parameters, size and thickness of weldment, thermal physical property of metal material and joint form.
Using small welding parameters, such as reducing welding current and increasing welding speed, can reduce the width of heat affected zone.
The width of heat affected zone varies with different welding methods.
The total width of heat affected zone of electrode arc welding is 6mm, that of submerged arc welding is about 2.5mm, and that of gas welding is about 27mm.
Methods for controlling and improving the properties of welded joints
1. Material matching
Material matching mainly refers to the selection of welding materials.
For low-carbon steel, low-alloy high-strength structural steel and low-temperature steel, the composition of weld metal and base metal is generally not required to be the same, but the mechanical properties are required to be the same as the base metal.
For heat-resistant steel and stainless steel, in order to ensure that the weld has high temperature performance and corrosion resistance similar to that of the base metal, the chemical composition of its welding material should be roughly the same as that of the base metal.
2. Control the fusion ratio
During fusion welding, the percentage of the melted base metal in the weld metal is called the fusion ratio.
The calculation formula of fusion ratio is:
r = Fm/(Fm+Ft)
r – fusion ratio;
Fm – cross sectional area of molten base metal:
Ft – cross sectional area of filler metal in the weld.