Alternative Methods for Determining Preheat and Interpass Temperatures

Preheating, maintaining interpass temperatures, and postheating, when needed, can consume considerable shop or field production time with costs for labor, fuel gases, and electricity. However, alternative methods for determining preheat requirements, when applied with today’s cleaner, lower-carbon steels and the use of H4 and H8 low-hydrogen filler metals, can easily achieve time and cost savings, as well as environmental benefits.

Steel fabricators and erectors typically rely on prequalified welding procedure specifications (WPSs) and use AWS D1.1, Structural Welding Code — Steel, Table 5.8, “Prequalified Minimum Preheat and Interpass Temperature,” as cited in Clause 5.7. As an alternative, Annex B, “Guidelines on Alternative Methods for Determining Preheat,” a normative annex, provides two methods to determine preheat and interpass temperature requirements based on the steel’s composition and thickness, the filler metal’s diffusible hydrogen, and an assumed level of joint restraint. However, it is rarely used, perhaps because of its perceived complexity, so the potential time and cost savings are lost. 

Another source, AWS D14.8M:2009 (ISO/TR 17844:2004 IDT), Standard Methods for the Avoidance of Cold Cracks, provides four methods to determine the required preheat temperature. Two methods (CE and CET) are based on European standards, one (CEN) is based on a Japanese standard, and one (Pcm) is based on Annex B of AWS D1.1. All four methods have been relied upon globally and used successfully for decades.

AWS D1.1 Table 5.8 has been the industry’s go-to reference. Its use has been driven by its simplicity and the advantage of utilizing prequalified WPSs, saving the cost and effort of procedure qualification testing. Category A can be used for lower-strength structural steels when using non-low-hydrogen shielded metal arc welding (SMAW) electrodes. Category B includes the majority of the commonly used structural steels (e.g., ASTM A572, up to 55 ksi specified minimum yield strength [SMYS]). Group C includes steel in the 60 to 70 ksi SMYS range. Group B and Group C require low-hydrogen electrodes that can range up to the H16 classification. Group D reduces the classification limit to a maximum of H8 and includes a limited number of steels (e.g., A913, from 50 to 65 ksi SMYS). Group E addresses a newer steel, ASTM 1066, a thermo-mechanically controlled processed steel, in Grades 50 through 65, when using filler metals classified H8 or lower. Quenched and tempered steels and special steels, such as ASTM A1043, must be qualified by test and are listed in AWS D1.1 Table 6.9.

Running the Numbers

The alternative methods consider the actual welding conditions, including steel composition, filler metal diffusible hydrogen level, part thicknesses, and heat generated during welding. Preheat and interpass temperatures can be dramatically lower when running the numbers rather than relying on a table that conservatively assumes a worst-case scenario for composition, diffusible hydrogen, and other factors.

The AWS D1.1 Annex B guideline provides two options: a heat-affected zone (HAZ) hardness control method limited to fillet welds and a hydrogen control method. Carbon equivalents are to be determined or assumed, and Figure B.1 of Annex B provides three zones to further guide the method to be used. Tables and/or figures, determined by method, can be used to determine the minimum preheat levels. Although relatively easy to understand and use, the tables can require significant increases in preheat requirements for nominal increases in thickness or Pcm. Basic but limited guidance is provided regarding levels of restraint.

AWS D14.8M provides four methods, including the above Annex B approach. It is often helpful to use all four methods (when applicable) to determine and verify the appropriate level of preheat and interpass temperatures. It should be noted that the document is in SI units only, so conversions are necessary for calculations and for the use of the tables. However, many of the results are provided through figures rather than extensive calculations, and using an overlay containing U.S. customary values for the vertical and horizontal axes of the figures, replacing the SI values provided, is quite helpful and a time-saver. 

It should also be noted that the calculation of heat input uses a thermal efficiency factor, which multiplies the results of the typical calculation using volts, amps, and travel speed by a factor of 0.8 for SMAW, flux cored arc welding (FCAW), and gas metal arc welding (GMAW) and a factor of 0.6 for gas tungsten arc welding and plasma arc welding. Only submerged arc welding (SAW) has a thermal efficiency factor of 1.0. For more explanation, see European standard EN 1011-1:2009, Welding — Recommendations for welding of metallic materials.

AWS D14.8M Chapters

Chapter 2

The CE method in Chapter 2 of AWS D14.8M uses critical hardness to avoid HAZ cracking. It is based on extensive HAZ hardenability studies and cracking tests in joints with thicknesses ranging from ¼ to 4 in. and applies to carbon, carbon manganese, and fine-grained and low-alloyed steels. The CEIIW equation, using seven elements, is used but does not consider added boron. It is applicable to the SMAW, self-shielded FCAW, gas-shielded FCAW, GMAW, and SAW processes and considers five filler metal diffusible hydrogen levels ranging from 3 to over 15 mL per 100 g of deposited weld metal. Note that AWS filler metal standards are categorized as H4, H8, and H16. The combined thickness of all parts being joined by the weld, rather than the thickness of the thickest part, is used. Figures A through M incorporate the considerations of filler metal diffusible hydrogen (≤ 3 to > 15), combined thickness, and heat input, providing the required preheat value for the given condition. 

Chapter 3

Chapter 3 contains the CET method, based primarily on y-joint (Tekken) weld cracking tests using groove welds in butt joints, and also includes fillet weld tests. Table 9 provides a range of 11 elements for which the CET method is applicable, including niobium (columbium), nickel, titanium, vanadium, and boron; however, the CET equation considers only six major elements. Plate thickness is limited to 3.5 in. and does not use the combined thickness of the joined elements as used in the CE method. Heat inputs as high as 100 kJ/in. are included. Chapter 3 provides equations (in SI units) that can be used to calculate a preheat value based on the actual condition. Caution is warranted to ensure that all conversions to U.S. customary units are done correctly. However, a series of individual figures is provided to determine your needed preheat, also allowing you to check your converted equations, if used, one step at a time.

Chapter 4

Chapter 4, the CEN method, was developed in Japan using y-groove testing and is particularly helpful when considering modern microalloyed steels. It adds a modifier to the basic seven-element carbon equivalent equation to better consider the benefits of lower carbon content. The applicable thickness range is 3/8 to 8 in. The method includes steels and filler metals to 110 ksi and effective heat inputs up to 125 kJ/in. A factor of 0.9 is used for SAW, rather than 1.0, as used for the CE and CET methods. Solutions for the CEN method are determined graphically, starting with a master curve followed by adjustments based on heat input and the calculated value of CEIIW, filler metal diffusible hydrogen, and restraint level (slit welding, repair welding, or typical welding).

Chapter 5

Chapter 5 is essentially the same as AWS D1.1 Annex B. In addition, AWS D14.8M includes an extensive and informative Annex A that describes and illustrates different results when using the four methods, including consideration of steel yield strength, plate thickness, heat input, and filler metal diffusible hydrogen level.

Summary

Using AWS D1.1 Table 5.8 is applicable when the prequalification of Chapter 5, Part F, is chosen by the fabricator or erector. AWS D1.1 Clause 6.8, “Essential Variables,” Subclause 6.8.4, “Preheat and Interpass Temperatures,” provides guidance on using Annex B or other methods to determine preheat and interpass temperature requirements. AWS D14.8M is an excellent source of other methods. Qualification testing is still required using preheat and interpass temperatures lower than those listed in Table 5.8. The number and cost of tests can be minimized by planning, and such testing can dramatically reduce the costs of time, labor, and energy when compared to relying on Table 5.8 alone, particularly when welding thick material with lower carbon equivalents and using processes and filler metals with low levels of diffusible hydrogen. 

This article was written by Robert E. Shaw Jr., P.E. (president of Steel Structures Technology Center Inc., Howell, Mich.) for the American Welding Society.