This article explores the following four aspects of preheat and interpass temperatures in code welds:
- In brief – what is preheat temperature, what is interpass temperature.
- Where exactly on metal surface should these temperatures be checked while welding?
- How to check them?
- Difference between the two.
Please note that most of the content of this article, like a bulk of this website, is centred around ASME Section IX. Also, the relevance of some of the ideas discussed here might be more to CS and LAS metals.
What Are Preheat And Interpass Temperatures
Preheat: is the application of heat to a weld joint before start of welding. It serves the purpose of slowing down the rate of cooling of weldment; thus allowing the hydrogen in the weld metal to diffuse out and thus avoid cold cracking. Preheat temperature is the minimum temperature specified on WPS which the welder must follow in order to ensure preheat.
Interpass temperature: During the welding, before start of each pass – temperature of weld joint must be checked, and regulated according to the WPS. The purpose of this monitoring is to maintain influence on the microstructure/ properties of the weld metal.
Note: This article does not explore what effect interpass temperature has on the properties of weld metal.
Where Should Preheat And Interpass Temperature Be Checked?
This question often does rounds in my shops. Where should preheat and interpass temperatures be checked?
On the weld metal? On the surface of weld edge preparation (WEP)? On the face of the joint? How much away from the edge? Let’s see.
Preheat Temperature
The definition of this term, given at QG-109 of Sec IX, contains pointers to this question. The definition indicates that for single pass welds – the preheat temperature is the temperature of the weld edge just prior to welding. In other words, the temperature is required to be checked on weld edge preparation (WEP).
The definition goes on to say that, for multi-pass welds, preheat temperature is the temperature in the section of the previously deposited weld metal, just before each pass is to be deposited. That is, the temperature is to be checked on the weld metal.
This rule however, it must be noted, is not the same as that adopted by the ISO codes. The ISO 13916 specifies that if the thickness of base metal is less than 50 mm, the temperature must be checked at a distance of 4T from the WEP, where T = thickness of base metal, but need not be more than 50 mm. If the base metal thickness exceeds 50 mm, the temperature must be checked at a distance of at least 75 mm away from the WEP in all directions, unless something else is specifically agreed with inspector/ customer.
The ISO 13916 also specifies that the temperature should be checked on the opposite face of the face which his being exposed to the source of heating, as far as possible. If not possible, temperature may be taken after the removal of source of heat (imagine a gas flame) after elapse of some time – which is to be at least 2 min/25 mm of base metal thickness.
Interpass Temperature
The Section IX definition of interpass temperature says that it is the temperature in the previously deposited weld metal or adjacent base metal (typically within 25 mm) just before laying of next pass. In other words, the temperature is to be checked on the base metal itself.
The ISO 13916 specifies the same location as the preheat temperature, for interpass temperature too.
I have summarized the above rules in the following table:
| ASME Section IX | ISO 13916 | |
| Preheat temperature | On the weld metal | If T<50 mm, check at a dist. Of 4T but need not exceed 50 mm. If T>50 mm, check at a dist. Of min. 75 mm |
| Interpass temperature | On the weld metal | Same as above |
How To Check Preheat & Interpass Temperatures
Codes do not specify any mandatory method for checking preheat and interpass temperature. So, any suitable method may be used. A commonly used method in the industry is thermos-chalks (also called as thermal crayons in some places).
A chalk comes with a rating of, say 150°C. If it is rubbed on a metal surface having any temperature above 150°C, the chalk will melt and evaporate. So chalks are a good tool for confirmation checks. Welder can keep a chalk for preheat and another for interpass in his pocket, and check periodically to confirm that his weld meets temperatures specified on the WPS. Chalks are a cool method of checking temperature for production welds.
Other more sophisticated methods include non-touch equipment such as pyrometers, thermocouples, etc. In these methods, a reading is generally displayed on the digital screen; and are a less fussy, slightly faster, and more accurate way of measuring the temperatures. Specially, if you are making a qualification test coupon, these methods – in my opinion – are a better way to check the temperature as you can record the precise value of preheat/interpass for each pass.
Difference Between Preheat And Interpass Temperature
Although Section IX lays out definition for both these terms at QG-109, the difference is not so obvious to the layman eye. Both definitions sound somewhat similar. Let’s see if we can make things a little clearer.
A good way of distinguishing between the two is to remember that preheat is specified as a minimum temperature while interpass temperature is specified as a maximum temperature. That is, throughout the duration of welding, the temperature in the weld region must not fall below preheat temperature, before start of any pass. Also, throughout the duration of welding, the temperature in the weld region must not be above interpass temperature, before start of any pass.
Another difference between the two is that while preheat Is an essential variable (and hence always applicable), interpass is a supplementary essential variable (and so – applicable only when impact toughness is a consideration).
Some confusion does arise when people accustomed to ASME Codes start using other codes. The understanding in above 2-3 paras is not found uniformly in other codes. For example, some codes/ standards specify that preheat is the temperature before start of first pass of the weld joint. Any temperature measured immediately before start of any of the subsequent passes is termed as interpass temperature.
Also, in other codes, the interpass may be specified as a minimum value too (this is generally equal to the preheat temperature). And so, in some WPSs written to AWS codes, one may find both minimum and maximum interpass temperatures specified on the WPS.
Effectively, the physical work on the ground would be the same with both the ASME WPS as well as the AWS WPS. Only, the semantics are different.
This website talks a fair bit on the subject of preheat and interpass temperatures for procedure qualifications to ASME Sec IX. One of main articles on this is this.
That is all. Thanks for reading. Comment below if you would like to say something.