Dear Experts,
Why Preheat?
One of the most troublesome features of the welding process is the danger of cracks developing in the weld metal or in the adjacent base metal.
Understanding the benefits of preheating can help you to produce "CRACK- FREE" welded joints under a wide variety of conditions.Pre heating before welding is a well recognized preventive measure against cracking.Preheating has been defined as"raising temperature of metal above the temperature of the surroindings before welding".Sometimes the entire part is heated,which is termed as general preheat,or only the vicinity of the weld is heated,is called as local preheat.
What causes cracks in weld?what factors result in conditions that favour weld cracking?
Welding usually involves greater and more violent changes of temoerature than other fabricating process.
There are two areas in a welded joint that may crack as a result of the welding operation.The first one is the HEAT AFFECTED ZONE[HAZ]adjacent to the weld.The second area that may crack under certain conditions is the weld metal itself.The weld metal used for most purposes has so low carbon content that it does not change its properties as markedly as the base metal even under the most rapid rate of cooling likely to occur in welding.Therefore, the weld metal itself is prone to cracking than HAZ.
It is the metal closest to the weld that cools most rapidly,& in many cases undergoes important changes in the structure and properties that may lead to cracking.
EFFECTS OF PREHEATING:The main reasons are as follows.
1)Eliminates or lessens the the danger of crack formation.
2)Minimizes hard zones adjacent to the weld.
3)Minimizes shrinkage stress.
4)Lessons distrotion.
5)Enhances diffusion of hydrogen from steel.
preheating therfore,increases the ability of the welded joint to withstand service conditions.It can also be viewed as a heat treating operation.Preheating prevents weld cracking in many ways.
A)It minimizes the formation of brittle martensite.
B)It causes austentite to transform very slowly to martensite.
C)It increases the diffusion and escape rete of hydrgen.
With Best Regards
Sampath
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