Is Decarburizing Always Part of Steel Production?

Only when steel is heated in the presence of air containing oxygen.

2C +O₂ --> CO₂

Think: "chemical reaction leaves visual evidence by depleting carbon from surface layer."

Decarburization is the absence of Carbon. You see the carbon is missing. Where did it go? What were the temperatures again? At what temperature will carbon materials oxidize?

2C +O₂ --> CO₂

milo

Heat treating involves 1) Heating the steel above the temperature where austenite forms, 2) Quenching the steel, and 3) Tempering the steel to reduce brittleness and produce the desired hardness.

The first and last steps are done in a furnace, which can have a carburizing, neutral, or reducing atmosphere. If the atmosphere is slightly carburizing or neutral, you will not get decarburization. If it's heated in air, you will.

When steel plate or structural shapes are rolled, they are generally red hot, and will lose carbon to the atmosphere.

It is also quite frequent that Iron oxide is present on the steel that is very often confused for carbon.

Yes. In general, the process happens mostly in air at high temperature. You're going to find some decarb, provided that the steel has enough carbon to lose (i.e. is not stainless, or low carbon steel). Some special tubes are processed in controlled atmosphere. That is a different story.

Are any steps of the steel tubes manufacturing done at high temperatures?

Is it done in air?

Please review my original post.

milo

I thing you post no#1 was clear, just needed a bit of reading. All other posts become non-mandatory after that. Why the posts are still flowing in ?

OP should just read it and understand.

Decarburisation is the part of the game - accelerated if the heating in above certain temperature- and you have to take special steps to avoid it (and the effectiveness of the method is cross-checked by the measurement of the thickness of decarb layer)

GA to the only relevant.

Can it be removed during susequent carburising & hardening in sealed quench furnaces.

So far we have failed to remove it.

check the process- where the oxygen is in contact with the hot metal before it cools down.

It may be - you are taking out of furnace while metal is still too hot

In case of a gas heated furnace, check the presence of Hydrogen which will draw out the C from steel. maintain a bit reducing atmosphere.

The quenching media also more often than not may result in a bit of decarburisation, only it will be very thin due to the less time the hot metal is in contact.

http://www.vacaero.com/Education-Training/Magazine-Articles/Vacuum-Hardening-High-Strength-Steels-Oil-Vs.-Gas-Quenching.html

Except that i can not at the moment think of any other reason.

It seems I have not been able to make myself understood.

We procure raw steel(low carbon steel) in seemless tubes form which has decarburising

layer in the bore.

Then we heat treat (carburise & hard) in sealed quench furnaces with LPG & METHANOL.

Even then decarb. layer stays.

Should it be avoided during soft stage or during heat treatment?

What is the thickness of the decarb layer (before and after ?) It is strange that the carbon migration not taking place-unless there is some inhibitor layer applied by the tube supplier- or the tube length is such that the carburising atmosphere is not penetrating inside. Of course you can check on the edge portion. Or some passivation or other treatment carried out though in normal circumstances they should not matter.

Thickness is 30-50 microns before and afterHEAT TREATMENT. The length of piston pin is 50mm.The tubes are received in 4-5mtrs. length and then cut to 50mm pieces.The decarb. is observed in 20% pieces.

Mssr. Ankush Jain:

You have received several excellent answers in general terms regarding the issue of decraburization.

You have not provided much in the way of relevant specifics.

For further assistance we need to know is the piston pin problem same as the manufacture of tubes problem? Same material????

WHat is the chemical compoosition of the tubes? what is the chemical compositiion of the piston pin material?

What is the decarb incoming? upon charge to furnace? Frequency and severity measures on the tubing ID before your furnace process?What stock removal have you taken from the OD? is their still scale present? If so yu will never carburise through scale it supplies its own oxygen and provides a barrier to diffusion.

What is the cycle and dew point of the carburise and quench furnace?

What is the process path prior to the carburise and quench furnace?

As sb has pointed out, their can be a number of things preventing carbon uptake on your parts, but we need to have facts presented systematically if it is us who have to solve this problem, rather than you from the general principles we have presented.

a fishbone diagram would isolate areas of materials, method, machine, and manpower as areas that contribute to the process. We know nothing about any of these save low carbon tube (we think) unknown decarb (except your process doesn't make it better) and unknown process ( except sealed carburize and quench furnace we think).

from the data presented we are not able to know whether the 20% figure is a reflection of the incoming material decarb problem or a resultant of the heat treatment process.

We have no comparatiive data for decarb after heat treat where there has been stock removal. That would tell us whether your process is more or less in control or not.

WE NEED DATA< not little glimpses and flashes as if it were being handed out by a strip tease.

I regret I have limited availability for email currently. however, a better description of the problem and process specifics will help you and others better identify the area to be investogated as most likely cause.

Please provide details of your process path.

milo "piston PIN is a TUBE?"

same material, same problem.Low carbon steel.Composition of tubes & piston pin.

c 0.12-0.18%

Si 0.15-0.35%

Mn 0.55-0.90%

P 0.03%MAX

S 0.03% MAX

Cr 0.85-1.25%

Mo 0.15-0.35%

30-50microns decarburising layer at ID & 0.35mm stock removal from OD. No stock

removal from ID.No scale is present.

The pins are carburised at 925degree C in furnace.95 mts carburise, 15 mts diffusion

at 925 degreeeC.Then intermediate gas cooling for 15 mts; soaking at 910 degreeC

for 45mts & then quenched in low speed quenching oilmaintaed at 40-50degreeC.

Prior to furnace operation, tubes are cut in desired length & corners chamfered.20% figure is reflection of the incooming material & not heat treatment process.As said earlier,there is no stock removal in bore after or before heattreatment