PWHT furnace conditions

[Zhilla]

Dear All

Is there any Standard procedure that covers the furnace conditions for PWHT in a way that satisfies the ASME BPVC requirements of UCS-56 and UW-40? Or do the professional companies that perform PWHT have their own procedures? My concern is about the proper location of thermocouples and the right spacing of them. Also the appropriate number and location of the burners. The code refers to WRC-452. Is it enough?

Thank you all in advance.

 

[metengr]

Procedures and furnace surveys for PWHT are fabrication process details that are not specifically addressed by the Code. There are some guidelines that have recently been adopted by ASME B&PV Code for CSEF steels.

 

[racookpe1978]

Our company - who regularly contracts different field service heat treatment companies (not furnace heat treatment) - has specific requirements for T/C locations, PWHT and preheat spec's, and cooldown rates in our own procedures. So, for us, it doesn't matter - we have to follow the company processes.

 

[weldstan]

There are vast differences in operating and surveying of furnaces by PWHT providers. In you review of potential providers, I would advise that you obtain information as to recent furnace surveys and the data obtained therefrom. I would also recommend that you provide specifications as to T/C placement in you P.O. as well as heating and cooling rates when not specified by Codes or when you desire slower rates than prescribed by Codes. You may also wish to address furnace atmosphere requirements as well.

 

[racookpe1978]

Adding to the above: YOUR survey and audit" for YOR products cannot just rely on the HT company's temperature or area surveys, for either a batch "load and heatup and soak and cooldown batch unload" process, or a continuous run conveyor process.

You need to place the thermocouples on and inside and underneath YOUR actual parts (do several parts in each of the HT company's ovens!) and and monitor the results over time: DO the parts heat up at the right rate? Does ALL of a complex part get to the soak temperature for the right amount of time. Are any small sections too hot?

 

[Zhilla]

Thank you all for your replies. My concern comes mainly from the fact that one of our clients has asked me to explain in general how I can prevent carbon diffusion into the steel plate during heat treatment. I had not thought about this and have no answer to that. I know that there is the possibility of carbon diffusion into the surface of steel plates actually there is the process of carbon diffusion surface hardening for steels. But I cannot understand how it affects the carbon content of the steels during PWHT since the furnace is heated using flames that contain carbon. I was searching to see if there is a special procedure to prevent this phenomenon or if this is a concern in the first place. You see sometimes it is necessary to keep the carbon content to a minimum level to prevent future corrosion related problems.

 

[weldstan]

Combustion products mainly consist of H2O and CO2. You can add controls to provide for a slightly oxidizing atmosphere to prevent any carbon diffusion. Also review Gibbs free energy charts for discussion with your Client.

 

[metengr]

For subcritical PWHT you do not worry about carbon diffusion - carburizing from flames. What you do worry about is oxidation of the steel surface. Although, again for subcritical PWHT it is a minor concern for boiler and most pressure vessel pressure retaining components. You can investigate furnace atmospheres to avoid surface oxidation.

 

[Zhilla]

Thank you for your replies.

The head in one end of the vessel is not welded when the PWHT is being performed. Since we are worried that without this head there may be excessive deflection, we want to use some spider beams to protect the shell from getting oval. Is it not a problem as per the code to weld these beams and after PWHT cut the spider beams?
Furthermore, I want you to know that I am not worried about carbon diffusion in the carbon steel but about carbon diffusion in 304L stainless steel. Since the content of carbon is less than 0.03 per cent, I thought that this may increase the possibility of carbon diffusion.

 

[r6155]

Try to design spider beams without welding to the shell.

Regards
r6155

 

I have observed that there seems to be a long tradition of leaving heat treatment contractors to their own devices, including performing their own hardness testing (with the crudest possible methods, and in violation of the most basic norm of QC - you cannot inspect your own work).
We are now in an era where PWHT requires engineered procedures, which is reflected in documents like AWS D10.10 and WRC-452 you mentioned, and practices and procedures such as attested to by racookpe1978. While the BPV Code offers little guidance, it behooves you to apply 'engineering judgment' and address all concerns around heat treatment - distortion, structural integrity, effective relief of stresses, avoidance of undesirable metallurgical alteration, protection of exposed surfaces where necessary, and of course conforming to Code.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

zhilla, Are you solution annealing the 304L after forming? These are high temps.
Or is this a lower temp process? (unless this is very low C <0.015% sensitization is still a risk)
You really only have two options, either HT in air and then blast and pickle to clean, or HT in dry hydrogen to keep the surface oxide free. I have seen furnaces that heat with products of combustion (no radiant tubes) and they introduce significant excess air to assure oxidizing conditions.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[pradipgoswami]

Hi Zilla,

PWHT Furnace calibration is done together with calibration of thermocouples and temperature recorders. Accuracy of these components typically decides the accuracy of any PWHT operation.
Look under paragraph 8.0 of AWS D-10.10 and other clauses. It talks about the various ways to calibrate above underlined components. While dealing with PWHT procedures I've come across the several ASNI standards applicable for calibrating thermocouples.

A good way to cross refer would be read through the manual of the PWHT contractor and verify the accuracy. Definitely certified PWHT vendors know , what they're up tp and what standards to be followed.
Thanks.

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
ca.linkedin.com/pub/pradip-goswami/5/985/299