preheating

[Souravbohray]

what is use of preheating ?
how can control rate of cooling by preheating ? by apply of heat how cooling rate can be control ?
by preheat hardness increase or decrease?

[CHITTARANJAN BHIWANDKAR]

Thanks/Regards,Preheating is the process applied to raise the temperature 

of the parent steel before welding. It is used for the following main reasons:

·         The slower cooling rate encourages hydrogen diffusion from the weld area 

by extending the time period over which it is at elevated temperature 

(particularly the time at temperatures above approximately 100°C) 

at which temperatures hydrogen diffusion rates are significantly higher than at 

ambient temperature. The reduction in hydrogen reduces the risk of cracking.

·         To slow the cooling rate of the weld and the base material, potentially resulting in 

softer weld metal and heat affected zone microstructures with a greater resistance 

to fabrication hydrogen cracking.

C.R.Bhiwandkar

Supreme Megastructures Pvt. Ltd.

Cell No.:-+919867139337

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[George Dilintas]

It is also used to control the distortion by reducing the thermal gradient since dL=α * L * dt

Too fast colling rate promotes martensite formation

If there is a risk of hydrogen entrapment, then the best way to remove it is the hydrogen back out treatment (200 degC)

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Dr. Georgios Dilintas,

Dipl. Ing. In Aeronautic and Space Engineering

Ph.D in Mechanics of Solids - Computational Mechanics

A.I.S, A.N.I, IRCA Lead Auditor

Welding, Stress Analysis, Corrosion, QA/QC, Failure Analysis, Risk Analysis

[Er Sourav Bohray]

Thanks for reply.

My next question is if cooling rate is fast it's mean cooling rate is high or sudden cooling due to which cracking is occur we do preheating.basically martensitic structure form.

Due to preheating material become more hard than brittle than cracking.

So we have to heat preheating for thickness above 25mm because cooling rate is high in higher thickness ?????

If hydrogen amount is high in weld than crack will come after cooling.so to get hydrogen out from weld during we give more to hydrogen to get out from weld by increasing cooling time by preheat???

How to decide for which material we have to use low hydrogen electrode because suppose we have material above 25 mm thick we use 6013 and 7019, 6011.so how to decide were to use 7018 ,hydrogen source is moisture it will come for all material y donor use 7018 for all material to reduce amount of hydrogen by use of 7018.

Please reply my inspector hold my WPS he wants reason y I m not using 7018 for all material

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[sandy]

Specific to your query, 

1. preheating is being performed to avoid martensite formation which is in turns depends on cooling rate vs. temperature. you can get details via TTT diagram.

2. As preheating will help to have to slow cooling, it will give ductile properties although the preheating temp depends upon of the alloying of the metal being welded.

3. You can refer various codes e.g. ASME section VIII, AWS D1.1, EN 1011-1, if you are not sure about preheat temp for specific material grade.

4. A hydrogen crack depends upon the amount of the diluted hydrogen in the weld metal and properties of base metal. You didn't specify the material grade involved.

5. Refer AWS  D1.1 table 3.1, 3.2 & 3.3. It will guide you on all your queries if your material grade is listed there.

Regards

Sandeep Kumar

[Er Sourav Bohray]

Why ss preheating is not required? If we weld ss material rapid cooling occur ? Rate of cooling is slow or fast or normal ? Stress generated or not ? Hydrogen will released properly without preheating ss ? Maternsite formation take place or not on fast cooling rate ? Ss become hard or not welding higher thickness as material suppose 80 mm thick plate ?

[sandeep kumar]

https://www.twi-global.com/technical-knowledge/faqs/faq-is-post-weld-heat-treatment-normally-required-when-welding-chromium-nickel-austenitic-stainless-steels/

read in detials. it will clear your doubts. try to understand the metallurgical changes occurs in different phases.