What Preheat Temperature is Required?

john2005

What Preheat Temperature is Required?
Welding codes generally specify minimum values for the preheat temperature, which may or may not be adequate to prohibit cracking in every application. For example, if a bed-to-column connection is to be fabricated with a low-hydrogen electrode made of ASTM A572-Gr50 and A36 jumbo sections (thickness ranging from 4 to 5 in.), then a minimum prequalified preheat of 225F (107 C) is required (AWS D1.1-96, Table 3.2). However, for making butt splices in jumbo sections, it is advisable to increase the preheat temperate beyond the minimum prequalified level to that required by AISC for making butt splices in jumbo sections, namely 350F (175 C) (AISC LRFD J2.8). This conservative recommendation acknowledges that the minimum preheat requirements prescribed by AWS D1.1 may not be adequate for these highly restrained connections.

When no welding codes are specified, and the need for preheat has been established, how does one determine an appropriate preheat temperature? As a basis for discussion, consider AWS D1.1-96, Annex XI: "Guideline on Alternative Methods for Determining Preheat'''' which presents two procedures for establishing a preheat temperature developed primarily from laboratory cracking tests. These techniques are beneficial when the risk of cracking is increased due to composition, restraint, hydrogen level or lower welding heat input.

The two methods outlined in Annex XI of AWS D1.1-96 are: (1) heat affected zone (HAZ) hardness control and (2) hydrogen control. The HAZ hardness control method, which is restricted to fillet welds, is based on the assumption that cracking will not occur if the hardness of the HAZ is kept below some critical value. This is achieved by controlling the cooling rate. The critical cooling rate for a given hardness can be related to the carbon equivalent of the steel, which is defined as:

(Mc Si) (Cr Mo V) @i Cu) CE = C N '' N '' 6 5 15

From the critical cooling rate, a minimum preheat temperature can then be calculated. (Blodgett''s paper entitled "Calculating Cooling Rates by Computer Programming'''' outlines a calculation procedure based on cooling rate, heat input, plate thickness, temperature at which cooling rate is critical, preheat temperature, thermal conductivity and specific heat.) It should be pointed out, however, that "although the method can be used to determine a preheat level, its main value is in determining the minimum heat input (and hence minimum weld size) that prevents excessive hardening'''' (Annex XI, paragraph 3.4,AWS D1.1-96.)

The hydrogen control method is based on the assumption that cracking will not occur if the amount of hydrogen remaining in the joint after it has cooled down to about 120F (50C) does not exceed a critical value dependent on the composition of the steel and the restraint. This procedure is extremely useful for high strength, low-alloy steels that have high hardenability. However, the calculated preheat may be too conservative for carbon steels.

The three basic steps of the hydrogen control method are: (1) Calculate a composition parameter similar to the carbon equivalent; (2) Calculate a susceptibility index as a function of the composition parameter and the filler metal diffusible hydrogen content; and (3) Determine the minimum preheat temperature from the restraint level, material thickness, and susceptibility index

john2005

How is Preheat Applied?
The material thickness, size of the weldment and available heating equipment should be considered when choosing a method for applying preheat. For example, small production assemblies may be heated most effectively in a furnace. However, large structural components often require banks of heating torches, electrical strip heaters, or induction or radiant heaters.

A high level of accuracy generally is not required for preheating carbon steels. Although it is important that the work be heated to a minimum temperate, it is acceptable to exceed that temperature by approximately IOOF (40C). However, this is not the case for quenched and tempered

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