Dear Mr. Sil,
Some additional points on Nelson Diagram and High Temperature Hydrogen service are as follows.
Nelson's diagram is the basic building block for good metallurgical design in high temperature hydrogen service. High temperature hydrogen attack (HTHA), or otherwise known as hot hydrogen attack, is a problem which concerns steels operating at elevated temperatures (typically above 400°C) in hydrogen environments, in refinery, petrochemical and other chemical facilities and, also in high pressure steam boilers.
HTHA is the result of hydrogen dissociating and dissolving in the steel, and then reacting with the carbon in solution in the steel to form methane. Methane pressure builds up, forming bubbles or cavities, micro fissures and fissures that may combine to form cracks
High temperature hydrogen can attack steels in two ways:
a. Surface decarburization.
b. Internal decarburization and fissuring.
In order of increasing resistance: carbon steel, C-0.5Mo, Mn-0.5Mo, 1Cr-0.5Mo, 1.25Cr-0.5Mo, 2.25Cr-1Mo, 2.25Cr-1Mo-V, 3Cr-1Mo, 5Cr-0.5Mo and similar steels with variations in chemistry are the materials recommended for HTHA service.For a specific material, HTHA is dependent on temperature, hydrogen partial pressure, time and stress. Service exposure time is cumulative.
Use alloy steels with chromium and molybdenum have increased carbide stability thereby minimizing methane formation. Other carbide stabilizing elements include tungsten and vanadium. 300 Series SS, as well as 5Cr, 9Cr and 12 Cr alloys, are not susceptible to HTHA at conditions normally seen in refinery units.
Hope this would clarify your query.If you have any real issues, feel free to share.
Thanks.
Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist & Consultant
Ontario,Canada.
Email-pgoswami@sympatico.ca,
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of jignesh.rana@Linde-LE.com
Sent: Thursday, August 04, 2011 5:33 AM
To: materials-welding@googlegroups.com
Subject: Re: [MW:12102] Nelson curve / hydrogen service
Original Nelson curve was prepared by G A Nelson in 194, using two parameters; operating temperature and partial pressure of Hydrogen with respect to HTHA ( High Temperature Hydrogen Attack.) This curve was first published by API in 1970 and then underwent several changes after noticed cases of HTHA.
Presently widely accepted specification against HTHA is API 941. Fig. 1 of API 941 defines the temperature limits of various materials against partial pressure of Hydrogen.
If Coincident point (temp and pH2) falls above particular material line, then HTHA is likely to happen for that material for these conditions.
With Best Regards,
Jignesh R Rana
Linde Engineering India Pvt. Ltd.
This e-mail is confidential. If you are not the intended recipient, please notify us immediately. You should not disclose its contents to any other person nor use it for any purposes.
From: pradip kumar sil <pradipsil@gmail.com>
To: materials-welding@googlegroups.com
Date: 04-08-11 13:43
Subject: [MW:12101] Nelson curve / hydrogen service
Sent by: materials-welding@googlegroups.com
Dear everybody,
Can anybody tell me how can select material for hydrogen service AS PER NELSON CURVE
Regards
Pradip
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