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A Spreadsheet Model for Predicting Hardness in Carbon Steel Welds Disclosure Number: IPCOM000031463D
Publication Date: 2004-Sep-27
Document File: 38 page(s) / 4M

Publishing Venue

The Prior Art Database

Related People

Samuel Mishael: INVENTOR [+1]


Current models for predicting weld hardness are unreliable, especially for steels selected for high sulfur and H2S service. The prediction model presented here includes features which more accurately predict weld hardness, including welds in gas piping for H2S service.

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A weld can be susceptible to cracking from hydrogen embrittlement if it is too hard.  Sour services in particular require weld hardness be controlled to prevent cracking.  Staying within hardness limits is typically achieved by using the following methods;

·        Composition Control of the purchased components

·        Control of welding parameters

·        Use of Post Weld Heat Treatment

·        Qualifying a weld procedure by checking the hardness

·        Checking each production weld for cap hardness

These practices have worked for decades to provide reliable welds for many sour service plants around the globe.  However, it has also been recognized that there was no mathematical foundation to describe quantitatively how these practices interact so that predictions of hardness for a given set of conditions could be made.  It is also recognized that ASME codes for construction do not cover requirements for sour service. Operators must therefore add necessary requirements to assure welds will not crack in sour service.

Using the practices above, hard welds were found while qualifying welds for the TCO Sour Gas Injection Project.  This plant is designed for high pressures and pushed Carbon steel pipe making to its limit in terms of size, strength, wall thickness and weldability.  To explain the hard welds, a spreadsheet program was developed by ChevronTexaco to predict hardness based on composition, welding parameters and PWHT. 

The spreadsheet model has proved useful to explain observations made on actual welds in the KIO sour gas injection plant and qualification welds for the TCO sour gas injection project.  This report describes the spreadsheet model including several of the experiments and qualifications that were used to fine tune it.  It provides limits of applicability regarding composition and welding techniques.  Model accuracy is believed to be plus or minus 20 HV points, roughly 8% when trying to meet NACE hardness of 250HV.

However, all users should confirm results in actual welding tests and not rely on predictions.

A separate document, “Instructions For Use of the Weld HAZ Hardness Spreadsheet Model,” provides the mathematical foundation for the equations and methodology used. 

Predicting hardness is an ability provided by the spreadsheet model.  As such, the general practices listed above can be evaluated quantitatively.  This will aid a designer’s ability to assure hardness requirements for sour service are achieved.


This document provides background and instruction for the use of a weld hardness calculator.  This calculator was designed to assist in the evaluation of Carbon steel (CS) heats of material submitted to TengizChevroil (TCO) and Karachaganak (KIO) for use in construction of piping in Sour Gas Injection (SGI) Projects.  The program predicts hardness as welded and after PWHT.

This program is believed to be valid under the following limitat...