Inspectioneering
Inspectioneering Journal

The Power of Proper Post-Weld Heat Treatment

By Marc McConnell, P.E. at Pro-Surve Technical Services, and Tracey Maloney at Versa Integrity Group. This article appears in the May/June 2017 issue of Inspectioneering Journal

Introduction

The failure of this vessel weld joint was caused by improper PWHT (Post-Weld Heat Treatment) applied during vessel fabrication!” This is what a well-experienced API Inspector told me when I was just a young engineer. My internal instinct was to challenge the Inspector. How did he know what happened years ago when the vessel was fabricated? Fortunately for me, I didn’t vocalize my thoughts to that salty individual. I held my tongue (a rare event), until I could do some reading and research!

So here is what I learned after reading some articles. That “simple” process of PWHT is far more detailed than I thought. It’s not like putting a cake in the oven and waiting for the buzzer to go off. Things can go wrong and like the very smart Inspector said, improper PWHT can cause failures.

Welding and the Importance of PWHT

Welding is the process of melting base metal and weld metal together. Let me take a moment to elaborate on the three distinct components in the welding process (if you are not already familiar):

  1. weld metal
  2. base metal
  3. heat affected zone (HAZ), which is a narrow area between the weld and the base metal.

With the above in mind, a simple description of the science behind the weld will give us a better glimpse into the importance and potential challenges involved in PWHT:

  • Grain structures of the HAZ differ from those of the base metal because it has been heated to a temperature just below the melting point of the material and then cooled quicker than the adjacent base metal. The maximum temperature reached, time at temperature and cooling rate determine the grain structure of the HAZ.
  • Grain structures of the weld metal differ from the HAZ, because it too has been heated and cooled at a rate different from the HAZ and because it is a “casting” with a much different thermal and working history. With each successive weld bead, additional heat is applied to the weld below it, so each pass cools at a slower rate.

Unfortunately, mechanical, physical, and chemical properties within the HAZ are not always predictable or desirable and frequently cause problems. For example, in carbon steel and low allow steel, the HAZ can be very hard. As a result, this area can be brittle and commonly succumbs to various damage mechanisms (Caustic Stress Corrosion Cracking, Amine Stress Corrosion Cracking, sulfide stress cracking, stress-accelerated corrosion and many more mechanisms). In stainless steels and aluminum alloys, the corrosion resistance of the HAZ is diminished further than the adjacent base metal or weld metal. This can lead to a phenomenon called weld-decay, wherein the HAZ corrodes rapidly (i.e., stress accelerated corrosion/sensitization).

Proper post-weld heat treatment (PWHT) removes or minimizes the effects of these undesirable properties inherent in the HAZ. PWHT does this by changing the residual stresses and microstructure in the weld area.

Heat Affected Zone

This content is available to registered users and subscribers

Register today to unlock this article for free.

Create your free account and get access to:

  • Unlock one premium article of your choosing per month
  • Exclusive online content, videos, and downloads
  • Insightful and actionable webinars
GET STARTED
Interested in unlimited access? VIEW OUR SUBSCRIPTION OPTIONS

Current subscribers and registered users can log in now.


Comments and Discussion

Posted by Dana Baham on July 3, 2017
Location of temperature monitors is extremely... Log in or register to read the rest of this comment.

Posted by Thomas Langley on July 3, 2017
Great article! I met Marc at the API Summit Expo... Log in or register to read the rest of this comment.

Posted by Jacek Mielczarek on July 4, 2017
I am a welding engineer with years of PWHT... Log in or register to read the rest of this comment.

Posted by Marc McConnell on July 4, 2017
Have I received the comments on this article !!... Log in or register to read the rest of this comment.

Posted by Pablo Ramos on July 5, 2017
I agree and add another point . Considering that... Log in or register to read the rest of this comment.

Posted by Corey Herrick on July 6, 2017
Well done Mr. McConnell. I appreciate the time... Log in or register to read the rest of this comment.

Posted by Jess Lopez on July 10, 2017
I didn't quite understand how the Salty inspector... Log in or register to read the rest of this comment.

Add a Comment

Please log in or register to participate in comments and discussions.

Interested in equipment reliability & integrity? Sign up for weekly updates.

Inspectioneering Journal

Explore over 20 years of articles written by our team of subject matter experts.

Company Directory

Find relevant products, services, and technologies.

Job Postings

Discover job opportunities that match your skillset.

Case Studies

Learn from the experience of others in the industry.

Event Calendar

Find upcoming conferences, training sessions, online events, and more.

Industry News

Stay up-to-date with the latest inspection and asset integrity management news.

Blog

Read short articles and insights authored by industry experts.

Acronyms

Commonly used asset integrity management and inspection acronyms.

Asset Intelligence Reports

Download brief primers on various asset integrity management topics.

Videos

Watch educational and informative videos directly related to your profession.

Expert Interviews

Inspectioneering's archive of interviews with industry subject matter experts.