Prequalification of WPS
According to Chapter 3 of AWS D1.1, the definition of pre-qualified WPS is the exemption of the WPS (Welding Procedure Specification) from being qualified through testing. Thus, the advantage of writing them without exceeding the prescribed limits of the standard will save money as no test coupons will be required and no third-party laboratories will be engaged for the mechanical testing.
Prequalified Welding Processes
Here are the following prequalified processes:
The reason why number 3 cannot be pre-qualified is that GMAW-S or short-circuiting transfer operates on a low current setting and can be very difficult to weld without causing any discontinuities due to the low heat input. You can qualify it though with the qualification tests but it cannot be pre-qualified.
Other welding processes can be pre-qualified as long as the prescribed limits of AWS D1.1 are met.
Also, in accordance with AWS D1.1, here are the following code-approved processes meaning the standard allows it but needs qualification per Chapter 4 of the aforementioned standard.
Base Metals and Filler Metal Requirements
There are only select base metals and filler metals that are allowed to be pre-qualified. AWS D1.1 Tables 3.1 (may differ depending on the AWS edition you have) shows this information.
The table and the highlighted information can be interpreted as follows:
For instance, you need to weld base metals of different group numbers as highlighted below. You need to weld ASTM A 36 to ASTM A 570 Grade 50, AWS D1.1 provides a tabulation to guide the creator of WPS on what filler metal will be used.
Since in our hypothetical scenario, ASTM A36 (Group 1 metal) with 58 ksi tensile strength will be welded to ASTM A570 Grade 50 (Group 2 metal) with 65 ksi tensile strength, the base metals strengths are undermatching (assuming that the note Table 2.3 are satisfied as specified in the table above). The filler metal that will be used is the one below the lower strength group, since the lower strength group is Group 1, we cannot go any lower thus use E-60xx or E-70xx but we have to use low hydrogen electrodes such as E-6018 or E-7018.
If for example, the ASTM A36 matches the strength requirement of ASTM A570 Grade 50 then, filler metal of either group can be used.
Prequalified WPS requirements
AWS D1.1 specifies the WPS requirements for different variables in Table 3.7 as shown below.
The way you need to use this table for example, the maximum electrode diameter can be used depends on the process and position of the weld. You may also notice in the table that some of the welding process like SAW for vertical and overhead welding that is left blank. The reason why is that SAW can never be used for vertical and overhead, it is just not applicable.
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Preheat and Interpass temperature requirements
Preheat reduces the amount of hydrogen in the base metal and reduces the susceptibility of cracking. Heating the base metals can help diffuse the hydrogen much easier.
It also helps slows down the cooling rate of the base metal at the heat-affected zone (HAZ). This is done prior welding to avoid the formation of microstructures that will embrittle the metal.
As you can see in the below table, there is a minimum preheat and interpass temperature that can be prequalified for the thickness specified, but why minimum?
Going through lower temperatures of preheat, may promote embrittlement and non-uniform distribution of heat at the weld-zone, thus a higher temperature can be prequalified.
Interpass temperatures can also be treated as the same as to not promote cracking and embrittlement at the weld interface.
Considerations for CJP and PJP
CJP means complete joint penetration wherein the joints have complete penetration while PJP means partial joint penetration. PJP does not mean it is a discontinuity, it just depends upon the designer's discretion. Though in accordance to the table below, there is a limitation to the weld size for PJP and its corresponding base metal thickness.
On the otherhand, there are samples of prequalified PJP joints as shown and there are indicated in AWS D1.1 as well.
Limitations of WPS Variables
Changes on the following parameters beyond those written on the WPS shall be considered essential changes (by the way when writing prequalified WPS, you still need to identify the essential variables):
For example, when the shielding gas composition changes from 85% argon - 15% carbon dioxide to 100% CO2 then you need to write a new WPS for that. Also, a change in welding process will need a new WPS to be written.
Final note, one might think that a prequalified WPS may not be documented but any WPS must be documented and written as it is a guide for the welder how to create a sound weld for that specific application.
If you want to watch this guide instead, I have a video detailing this prequalified WPS
If you want to know more about welding inspection and pass the CWI exam, I have a welding introductory course for the fundamentals part of the CWI just click the link here and it will take you to the course it's free! If you love more free stuff and want to know more, you can also check out my videos here regarding the fundamentals of WPS and PQR . You need the fundamentals of WPS and PQR to reinforce your understanding of prequalification and qualification of welds
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7 个月Bartolo Allocca thank you for sharing to your network!