Question

Discuss the cracking phenomenon under the following heading in fusion-welded superalloys. 1. Definition
2. Types
3. Microstructure evolution and phase transformations

4. Remedies

Discuss the following weld imperfections and give remedies for each
1. Porosity

2. Cracks/fissures

Answer 1

1. Definition of cracking

Cracking is welding defect in which micro/macro cracks are observed in weld metal zone or in base metal zone, mainly due to any of following.

a) Structural stress in metal ( Formation of martensite)

b) Heavy shrinkage

c) Extra high amount of sulphur, Phosphorous or carbon in the metal

d) Excessively rigid clamping of parts being weld

e) Presence of gases in weld metal

2. Type of cracks

Welding cracks are of following types

a) Micro cracks

b) Macro cracks

c) Fissures

3. Microstructure evolution and phase transformations

Every weld process accompanies heat treatment process due to high temperature involved in welding process, This heat treatment process affects micro structure of metal.

Weld zone (WZ) comprises of eld metal zone (WMZ) and Heat affected zone (HAZ)

WMZ After completion of welding process molten weld joint is allowed to cool at ambient temp. The solidification process is that similar to casting. Thus in this zone WMZ the grain structure has many characteristics of a casting. the solidification process begins with formation of columnar grains. These grains are relatively long. These are parallel to direction of heat flow and are coarse grains. These grains posses low strength toughness and ductility.

HAZ

HAZ can be devided into two Zone

HAZ next outward to WMZ is the glossy HAZ or overheated zone, here the temp of metal is somewhat less than melting point. Overheating causes severe grain growth which is very pronounced throughout this zone. The grains are coarse and reduced mechanical properties in comparison to base metal.

Next sub Zone of HAZ attains temp. just above critical temp and long enough to complete allotropic transformation to austenite. It is followed by cooling rate fast enough to prevent grain growth to taking place thus this zone having refine grain structure

4. Remedies

a) Preventing formation of martensite- More the hardened metal slower must be cooling rate, parts should also be pre heated to retard cooling rate.

b) To control cracks generated by shrinking Using as thin material and as little filler material possible. Pre-heating to minimise temp difference between weld and base metal.

c) To avoid cracking due to entraped gases we should avoid formation of gases by thoroughly cleaning the joint from moisture/oil/grease/paint. And absorption of gases from atmosphere should be avoided by proper shielding.

5. weld imperfections

POROSITY

Also called voids Blow holes and gas pockets weakens welds appreciably and acts as stress raisers from which cracks spread

Cracks/fissures

Fissers are wide cracks which emerge to surface of metal