preheating

You don't provide enough information to answer you question.

Why are you preheating the metal? To make your bed warm, to get it ready to weld, or to cook food with. Is the metal, iron, steel, copper, silver or what. How big and how thick is the piece. A round disk has different needs than a piece 1/2 inch wide and 10 feet long. If you have a 2 x 4 foot sheet of 1020 steel it makes a lot of difference if the thickness is such that it weighs 3 pounds or 300 pounds.

A 1 foot square of of i
Inconel .125 inches thick needs a great deal less heat than a sheet of copper for the same size regardless what you do with it.

Gordon

Hey Guest,

Try saying "Please" and "Thank-you" it only takes a couple of seconds. People might respond a bit quicker if you ask for a favour instead of demanding an answer. Most people that are in CR4 are experienced and/or highly knowledgeable professionals. They take valuable time out of their day just to help people with questions and problems that they might know the answer to or be able to direct you in the right direction.

So have a bit of respect and you might find the answer your looking for.

Courtesy costs nothing but arrogance will get you nothing.

I totally agree with you.

It is a pity that politeness disappeared within the last years. It developed over thousands of years as a way to live in a more complex society. May be it will be good not to take into consideration questions put so as we would be obliged to answer them.

Now, where are you from, my dear? What is all this new racket? I do hope you benefit from the contribution the others have made without any fuss ! May be you should refrain from answering, who knows we shall benefit more than anything else. God bless all types.

Nice comment!

I thought that every body has the freedom to give an opinion.

You are free to appreciate lack of courtesy and others do not.

This is not a reason to become aggressive.

It would be quite interesting to know where you are from since as a guest you are incognito.

hi,

I am looking for your kind guidence and help.I would like to ask to pl forgive me for my arrogance or u can say ignorence. I was in hurry.I am sorry.

Looking forward for your kind response.

Thanking you,

With Regards

Well done shaffimj,

Sorry, if I was a bit harsh with you the other day. I was having another bad day. Thank-you for replying back with an explanation. That's respect and I applaud you.

Now I just wish I knew something about your problem, but I'm sure that other people will be able to help you in any way they can.

Cheers Mate.

Pre heaing in fabriction is pretty long subject but always interesting. To put in a nutshell, Preheating is used in welding carbon steels and low alloy steels to reduce the hardness of the weld metal and heat affected zone and helps to eliminate hydrogen induced cracking ( cold cracking) in these areas.

Pre heating is also used in welding of metals, which are high conductors of heat such as copper, aluminium and their alloys and in the repair welding of cast iron parts.

some of the examples where pre heating is used:

1. While welding thick sections, cooling rate is important. as larger mass of the plate cools faster than thinner sections ( due to conduction) resulting in higher tensile and yield strength, higer hardness and lower elongation. This may lead to cracking at haz areas (under-bead ) due to certian local conditions/ stresses. Preheating helps to provide slow cooling and help to avoid above problems. so any carbon steel metals of 20 mm thick. and above in general gets benefited by pre heating and slow cooling.

2. While welding high carbon content steels ( medium, high) slow cooling will help to avoid brittleness at the weld zones. slow cooling is achieved by inducing pre heating of the joints. so any carbon steel metal with carbon above 0.30 % and coupled with thickness factor gets benefited by pre heating.

3. Alloy steel ( with 12 mm abdove thickness) with elements like Chromium, Molybdenum , Vanadium are benefited by pre heating. These elements are stress risers and slow cooling after welding ( through pre heating) will help to avoid cracking on the weld and at HAZ areas.

4. Cast iron, Aluminium , Copper and its alloys irrespective of thickness require pre heating and interpass temp. control to get good fusion and to avoid instant cracking ( hot cracking ) at the time of welding.

5. Pre heat and minimum inter pass temparature may be dictated by the composition of the steel , by the thickness of the metal to be welded, or by the degree of the joint restraint. Pre heating may be mandatory if the welding is done in accordance to the code .

6. ASME sEC. VIII , AISC specifications, AWS D 1.0, D1.1, AWS Bridge code 2.0, BS and other international codes give temparature details for various metals and coupled with thickness range.

7. Another point to note is to use low hydogen electrodes appropriate to steels used along with pre heating and post heat treatement where ever necessary will help to get crack free joints during fabrication, avoid pre matured failures ( bridge construction ) and have long working life.

8. A min. temp of 100⁰C ( med. carbon steel) average of 150 to 250⁰ C ( alloy steel) and max of 0f 300 to 400⁰ C ( Cu, Al and CI ) depending upon the steel should be considered.

SRIDHAR

hi sridhar,

I would like to thank you for your kind attention and information provided by you.

ThankYou.

With regards

Preheating is required during welding to assist in completion of the welded joints. The need for and temp. of preheat are dependent on:

1. Chemical analysis (carbon equivalent, CE),

2. Degree of restraint of parts being welded,

3. Elevated physical properties, and

4. Heavy thicknesses.

Carbon equivalent, CE is an empirical equation which has been developed to express weldability and to estimate the cracking susceptibility of a steel during welding and to determine whether the steel needs preweld and postweld heat treatment (PWHT) to avoid cracking.

To-date, several CE expressions with different coefficients for the alloying elements have been reported. The International Institute of Welding (IIW) carbon equivalent equation is :

CE = C+ Mn/6 +Ni/15 +Cu/15 +Cr/5 +Mo/5 +V/5

Where the concentration of the alloying element is given in weight percent.

Carbon is the element that most affects weldability. Together with other chemical elements, carbon may affect the solidification temperature range, hot tear susceptibility and cold cracking behavior of steel weldment. Steels with lower CE values generally exhibit good weldability.

CE (Wt. %)..Weldability cracking.......Heat treatment

< 0.45............ Unlikely...................None (preheat is option)*

0.45 - 0.60 ......Likely.................... Yes (200 to 400 ^oF)

> 0.60.............Highly probably ...... (400 to 700 ^oF) & PWHT

Other codes such as ASME BPVC states rules for preheating (Appendix R, ASME VIII, Div. 1) based on P-Number groups (weldability grouping).

For example : P-No. 1, Group Nos.1, 2, and 3:

(a) 175 ^oF (79 ^oC) for material which has both a specified max. carbon content in excess of 0.30%, and a thickness at the joint in excess of 1 inch (25 mm).

(b) 50 ^oF (10 ^oC) for all other materials in the P-Number.

And so on for every P-No., you will find how much preheating temp.

* As a personal point of view i prefer to use preheat for every material and each thickness for the following reasons :

1. To avoid at least the formation of H₂ from the invisible film of moisture located at metal surface.

2. To control rate heating & rate of cooling to get metal with sound mechanical properties.

hello sir ,

Thank you for your valuable information.

Can you pl tell me how we assign P number to any metal I mean p number is decided on what parameter UTS,Chemical composition or other.and same question with F-number.Looking forward for your guidence.

Thank you.

ASME BPVC assigns a P-No. for base metals. Each P-Number has a Group-Nos. ASME BPVC, Section IX (Appendix D), and Section II, Part D include a Tables for all base metals which contain a column specifying the P-No. & Group No.

P-No. is a base metal classification and assigned based on comparable base metal characteristics, such as composition, weldability, brazeability, and mechanical properties.

Group No. is a base metal grouping & classification within the P-No. for impact test requirements.

F-No. is a filler metal (electrodes & welding rods) grouping for qualification, and is based essentially on their usability characteristics which determine the ability of welders to make satisfactory welds with a given filler metals.

hi,

thank tou for your valuable information.

with regards

Many many assumptions made here today.

Preheating could also be proxy words for annealing or stress relief. Description of material, process, and what is meant by 'Fabrication' would indeed have saved people from wasting their time.

You're Welcome.

Milo

Preheat is performed for several reasons.

To reduce hydrogen inclusion.

To reduce thermal shock.

For service (such as H₂S service).

Code requirements.

For some materials it is imperative (P91 - P5A)

Reduce hardness as welded.

For thickness (to reduce the cooling rate on thick weldments)

Don't forget that if you preheat you may be required to Post Weld Heat Treat - stress relieve)