What is Preheat?

john2005

haha,today i took an article for what preheat is here.
it is vailable for us to improve english on welding

by Scott Funderburk, Engineering Services, The Lincoln Electric Company, May, 1998.


Preheat - Introduction
This new department is a response to our readers and was inspired by a discussion regarding the need to emphasize the significance of welding engineering fundamentals. Our purpose here is not to reveal "ground-breaking material; rather, the goal is to re-examine a series of topics essential to the science, mathematics, and technology of welding. In this inaugural column, we will explore the purpose and importance of preheat and its role in the fabrication of welded members.

What is Preheat?
Preheating involves heating the base metal, either in its entirety or just the region surrounding the joint, to a specific desired temperature, called the preheat temperature, prior to welding, Heating may be continued during the welding process, but frequently the heat from welding is sufficient to maintain the desired temperature without a continuation of the external heat source. The interpass temperature, defined as the temperature between the first and last welding passes, can not fall below the preheat temperature, Interpass temperature will not be discussed further here; however, it will be the subject of a future column. Preheating can produce many beneficial effects; however, without a working knowledge of the fundamentals involved one risks wasting money, or even worse, degrading the integrity of the weldment.

Why Preheat?
There are four primary reasons to utilize preheat: (1) it lowers the cooling rate in the weld metal and base metal, producing a more ductile metallurgical structure with greater resistant to cracking; (2) the slower cooling rate provides an opportunity for any hydrogen that may be present to diffuse out harmlessly without causing cracking; (3) it reduces the shrinkage stresses in the weld and adjacent base metal, which is especially important in highly restrained joints and (4) it raises some steels above the temperature at which brittle fracture would occur in fabrication. Additionally, preheat can be used to help ensure specific mechanical properties, such as notch toughness.

When Should Preheat be Used?
In determining whether or not to preheat, the following array of factors should be considered: code requirements, section thickness, base metal chemistry, restraint, ambient temperature, filler metal hydrogen content and previous cracking problems.
If a welding code must be followed, then the code generally will specify the minimum preheat temperature for a given base metal, welding process and section thickness. This minimum value must be attained regardless of the restraint or variation in base metal chemistry; however, the minimum value may be increased if necessary. An example is illustrated in the next section.

When there are no codes governing the welding, one must determine whether preheat is required, and if so, what preheat temperature will be appropriate. In general, preheat usually is not required on low carbon steels less than 1 in, (25 mm) thick. However, as the chemistry, diffusible hydrogen level of the weld metal, restraint or section thickness increases, the demand for preheat also increases. There are several methods to determine the required preheat temperature for a given base metal and section thickness that will be discussed in the next section

wrx0124

不错的英文资料，谢谢楼主分享，有学习价值

fiber

不错，值得学习，对英语的提高也有帮助。

xuyongguang

还真有点意思。望常常发表一点。