Heat affected zone

JAY PRAKASH

MTECH 3RD SEM

16 MEIM 061

GD-1368

MECHANICAL DEPARTMENT ZHCET AMU ALIGARH

The Heat-Affected Zone (HAZ)

The weld interface, which is also referred to as mushy zone,

is a narrow zone consisting of partially melted base material

which has not got an opportunity for mixing. This zone

separates the fusion zone and heat affected zone.

Refers to a non-melted area of metal that has experienced

changes in its material properties as a result of exposure to high

temperatures.

The alterations in material properties are usually a result of

welding or high-heat cutting procedures.

It can often be recognized by a series of brightly colored bands,

also visible near welds. Colors are caused by surface oxidation,

and are an approximate indicator of the temperature reached by

the metal:

These colors, also called "Heat tint", depend on four factors:

Steel chromium content: this metal increases the material

resistance to oxidation, therefore colors will be less intense .

Oxygen level: during welding, usage of protective gas and

electrode coating can reduce the coloration because they partly

shield metal from oxidation;

The rougher: the surface, the faster it oxidizes, causing darker

colors .

Causes of the Heat-Affected Zone

Heat affected zones can be of varying size and strength. The

extent and magnitude of the HAZ is inversely proportional to

the thermal diffusivity and cooling rates of the material:

Where thermal diffusivity is high, the material cooling rate is

high and the HAZ is small.

Where thermal diffusivity is low, the cooling rate is slower and

the HAZ is larger

Cont..The extent and magnitude of property change depends primarily

on:

Base material

Weld filler metal

Amount and concentration of heat input by the welding process

Cont..The width of the HAZ is influenced by:

Cut speed - In general, faster speeds result in a smaller HAZ.

Amperage (when using plasma) - For a given thickness of

metal, a higher amperage (and consequently a faster cut speed)

results in a smaller HAZ.

Type of metal being cut - Increased temperatures and longer

cutting times will result in a wider HAZ.

Cont..All thermal cutting processes create a heat-affected zone in the

cut metal. The amount of heat inputted by the welding process

plays an important role:

Oxyfuel welding uses high heat input and increases the size of

the HAZ.

Laser and electron-beam welding give a highly concentrated,

limited amount of heat, resulting in a small HAZ.

Arc welding falls between these two extremes.

Change in microstructure HAZ

Near the fusion boundary a course grain structure is formed. It

is increase the ductility.

After the course grain structure the fine grain structure is

formed it increase the strength of the material.

Effects of Heat-Affected Zones

Because the HAZ experiences sufficient heat for a long enough period of time, the layer undergoes microstructure and property changes that differ from the parent metal.

These property changes are usually undesirable and ultimately serve as the weakest part of the component.

the micro structural changes can lead to residual stresses, reduced material strength, increased brittleness, and decreased resistance to corrosion and/or cracking. As a result, many failures occur in the HAZ

Conclusion HAZ can not be removed in fusion welding process.

The residual stress is formed in the weld metal

It effect the mechanical property.

The micro structural changes can lead to residual stresses, reduced material strength, increased brittleness, and decreased resistance to corrosion and/or cracking.

As a result, many failures occur in the HAZ

Reference

http://nptel.ac.in/

https://en.wikipedia.org

https://www.aws.org/

Text book production engineering Dr. Swadesh kumarsingh