Preamble: this article has been published in the December 2015 issue of the Canadian Welding Association's Welding Journal.
- Spot Welding Acceptance Criteria
- Stud Welding Acceptance Criteria
- Asme Visual Weld Acceptance Criteria
- Welding Distortion Acceptance Criteria
- Visual Acceptance Criteria
- Welding Acceptance Criteria Examples
Acceptance criteria for Magnetic Particle Test (MT) (Refer Mandatory Appendix 6 (Page 412) of ASME BPVC Section VIII Div 1, 2017 Edition. Acceptance criteria are the same as that of Liquid Penetrant Examination) The following terminologies have been used to explain the acceptance/rejection criteria for Magnetic Particle Test (PT). A welding procedure is being qualified using ASTM A709 Grade 100W Type E Plate. The thickness is 1-1/2' and the welding is done in the 2G welding position using SMAW process. Assume all requirements are successfully met. For which of the following welding positions would the individual who welded this qualification be qualified?
When welds fail in service, the extent of failure analysis performed is dependent on how it may have affected life, property value or production. These investigations provide feedback that over time has influenced the development of weld quality acceptance criteria for various types of welded structures. This article suggests the why, how and what of inspecting, evaluating and trying to improve weld quality.
To play a role in preventing the failure of welds, the Inspector should have a clear understanding of these aspects of weld quality:
- What aspects or characteristics of a weld contribute to its fitness for purpose,
- What are the available and appropriate inspection and/or test methods that should be used to assess those quality characteristics,
- What is the relevant acceptance criteria and how is it applied in evaluating the magnitude of various types of weld quality issues,
- Knowing the process variable that contributes to defective welding enables the Inspector to play a role in preventing those defects in future work.
1. What can be inspected and/or tested
Welds can fail in service due to defective characteristics in these categories:
a) Structural Soundness Discontinuities: Imagine looking through a weld and if you see anything other than weld metal, this is a loss of soundness. Things like porosity, cracks and lack of fusion should not be in the weld and are typical discontinuities in this category.
b) Geometric Discontinuities: Welds can fail because they are the wrong size or shape. Undersize welds are typical geometric discontinuities but welds that are oversize can also provide negative consequences due to the additional heat input causing more distortion. Shape also needs to be considered as sharp changes in direction are stress-risers which is why most codes also have limits on the maximum height of reinforcement permitted in groove welds.
c) Property Discontinuities: Welds that are the correct size and shape and pass ultrasonic inspection with flying colours can still fail in service if the mechanical and/or chemical properties do not match the application conditions and loading.
2. Methods of inspection and testing
Weld quality can be inspected for the first two categories using appropriate NDE methods and/or gauges during visual inspection. Weld quality for property characteristics however is not inspected but instead is verified by first qualifying a particular welding procedure and then monitoring production welding to ensure procedures are followed. This verification will include mechanical and/or chemical testing to assure the weld meets required application properties.
The inspection method used is selected based on why welds are being inspected in regards to the consequence of failure. In some cases a simple visual inspection will suffice while in situations, it is accompanied by one or more NDE methods to examine the weld inside and out. Often, the method of weld inspection is specified by the relevant code or standard and in the absence of those requirements, may be specified by customers or the responsible Engineer.
In some quality assured environments, the inspection method is detailed in a documented procedure to assure reliable and consistent results. These documents list the steps of inspection in sequence and list requirements for variables, calibration, etc. Even something as simple as visual inspection is sometimes defined in documented procedures to ensure proper inspection tools are used, suitable illumination, competent Inspectors and reporting requirements are met. Shown below are some various visual inspection tools that may be required for weld inspection.
3. Weld quality acceptance criteria
Inspectors examine welds looking for 'discontinuities' which are changes in the intended structure or properties of a weld. Considering the structural soundness category of quality, anything other than clean metal within or beside the weld is considered a discontinuity. If found, Inspectors should not immediately reject the weld quality but instead evaluate the discontinuity found in comparison to the maximum permitted in the relevant acceptance criteria. Discontinuities that exceed the acceptance criteria are then identified as 'defects' and must be repaired or the item scrapped while discontinuities within allowable limits may be recorded as 'flaws' in an inspection report and do not require repair.
Ideally, welding acceptance criteria is defined in a tangible format with clear direction on what gets accepted vs rejected with different formats and combinations used based on the type of discontinuity. In some cases, statements are used such as 'no cracks' provide an easy route to the accept/reject decision. The rationale behind zero tolerance for some discontinuities like cracks is they are prone to propagation or growth with successive loading over time ultimately leading to complete failure.
In other cases, individual discontinuities must be measured with some type of dimensional tolerance then applied. Let's look at the ways to define porosity acceptance criteria listed below from a structural steel welding standard:
A magnifying glass and a visual guide showing these magnitudes of porosity would be useful in properly assessing the acceptance status of porosity. Even more challenging than porosity perhaps is the criteria to assess undercut shown below from this same structural welding standard. This complexity in assessing discontinuities such as porosity and undercut can sometimes lead to short-cuts in rejecting discontinuities that are in fact acceptable per code acceptance criteria. Undercut should be measured with a suitable gauge like the pit depth gauge illustrated earlier.
One last option that is sometime used for weld acceptance criteria is sketches or actual samples showing the maximum magnitude of a specific discontinuity permitted before rejection.
These examples illustrate some of the important information the Welding Inspector needs prior to beginning to assess weld quality which may need to be defined by the responsible Engineer:
- What is the relevant code and/or acceptance criteria?
- What is the nature of the loading on the welded structure?
- Are primary stresses along or across the weld?
A note of caution in applying acceptance criteria is that in some cases, the criteria to apply is conditional on the method of inspection or NDE. Another consideration is that inspection of some quenched and tempered HSLA steels must not be done till 48 hours after welding as these are more susceptible to hydrogen initiated cold cracking.
4. Preventing recurrence of failed welds
While identification and evaluation of welding discontinuities may be the primary function of a Welding Inspector, they can also play a role in preventing recurrence through root cause analysis and corrective action.
The ability of any welding process to make quality welds is dependent on the welding procedure used which has variables in the categories listed below. There may be some differences based on which welding process is being used and if it is performed manually or automatically.
Welding is a surprisingly complex process involving a wide range of variables that influence weld quality. When welds are defective, one of more variables not adjusted or controlled properly for the application has been the cause. The Inspector needs to analyze the discontinuity, investigate how it was welded and then problem solve to determine which variable(s) were likely to have caused the problem. When welds have been identified as defective and will be repaired, the Inspector should monitor the repair process carefully.
Comprehensive weld inspection is a much more detailed process than simply marking weld inconsistencies with a marker for repair. It requires knowledge including welding processes, metallurgy and inspection methods. To ensure this is done correctly, many codes including CSA, AWS and IIW require that Welding Inspectors demonstrate their competence through qualification certification.
When qualifying a welding procedure specification (WPS) for fillet welds per Clause 4 of AWS D1.1 Structural Welding Code you must only perform a single test. You must do macroetches in three locations as shown below.
Macroetch locations for welding procedure qualification of fillet welds.
It is assumed that if we can show root fusion at these three locations that our welding procedure is acceptable. Meaning it is capable of producing good welds.
However, when doing a welder performance qualification there is a difference in the test. The welder must stop at the middle of the test coupon, do a restart and finish out the weld. Instead of three macroetches only two are taken, each 2 inches from each end. The middle section of the coupon is then subjected to the fillet break test. The image below shows the location of the macroetches and the stop/restart.
A stop and restart near the middle of the weld is a requirement for the welder performance qualification test.
Spot Welding Acceptance Criteria
A macro etch gives us a clear indication of whether or not we achieved root fusion. Root fusion shall be achieved per AWS D1.1 in order for the welder to be qualified. This should be shown on both macroetches as seen below.
Macroetches show whether or not root fusion was achieve. it also provides details such as depth of root penetration, penetration depth onto the side walls, area affected by the heat (HAZ), size of legs, effective throat and more.
It is not necessary to use a camera that provides magnification as the one used for the images above, but it can help if achieving root fusion was in question. The red lines are added to clearly show the root of the joint (intersection of the red lines). Macroetches provide a lot of useful information, but they only provide information on single location of the weld. Move half an inch and the results may be different.
Since these macroetches are taken two inches from the start and two inches from the end we get reasonable assurance that the welder was able to maintain adequate travel speed, transverse and travel angles, steady contact tip to work distance and good technique.
In some quality assured environments, the inspection method is detailed in a documented procedure to assure reliable and consistent results. These documents list the steps of inspection in sequence and list requirements for variables, calibration, etc. Even something as simple as visual inspection is sometimes defined in documented procedures to ensure proper inspection tools are used, suitable illumination, competent Inspectors and reporting requirements are met. Shown below are some various visual inspection tools that may be required for weld inspection.
3. Weld quality acceptance criteria
Inspectors examine welds looking for 'discontinuities' which are changes in the intended structure or properties of a weld. Considering the structural soundness category of quality, anything other than clean metal within or beside the weld is considered a discontinuity. If found, Inspectors should not immediately reject the weld quality but instead evaluate the discontinuity found in comparison to the maximum permitted in the relevant acceptance criteria. Discontinuities that exceed the acceptance criteria are then identified as 'defects' and must be repaired or the item scrapped while discontinuities within allowable limits may be recorded as 'flaws' in an inspection report and do not require repair.
Ideally, welding acceptance criteria is defined in a tangible format with clear direction on what gets accepted vs rejected with different formats and combinations used based on the type of discontinuity. In some cases, statements are used such as 'no cracks' provide an easy route to the accept/reject decision. The rationale behind zero tolerance for some discontinuities like cracks is they are prone to propagation or growth with successive loading over time ultimately leading to complete failure.
In other cases, individual discontinuities must be measured with some type of dimensional tolerance then applied. Let's look at the ways to define porosity acceptance criteria listed below from a structural steel welding standard:
A magnifying glass and a visual guide showing these magnitudes of porosity would be useful in properly assessing the acceptance status of porosity. Even more challenging than porosity perhaps is the criteria to assess undercut shown below from this same structural welding standard. This complexity in assessing discontinuities such as porosity and undercut can sometimes lead to short-cuts in rejecting discontinuities that are in fact acceptable per code acceptance criteria. Undercut should be measured with a suitable gauge like the pit depth gauge illustrated earlier.
One last option that is sometime used for weld acceptance criteria is sketches or actual samples showing the maximum magnitude of a specific discontinuity permitted before rejection.
These examples illustrate some of the important information the Welding Inspector needs prior to beginning to assess weld quality which may need to be defined by the responsible Engineer:
- What is the relevant code and/or acceptance criteria?
- What is the nature of the loading on the welded structure?
- Are primary stresses along or across the weld?
A note of caution in applying acceptance criteria is that in some cases, the criteria to apply is conditional on the method of inspection or NDE. Another consideration is that inspection of some quenched and tempered HSLA steels must not be done till 48 hours after welding as these are more susceptible to hydrogen initiated cold cracking.
4. Preventing recurrence of failed welds
While identification and evaluation of welding discontinuities may be the primary function of a Welding Inspector, they can also play a role in preventing recurrence through root cause analysis and corrective action.
The ability of any welding process to make quality welds is dependent on the welding procedure used which has variables in the categories listed below. There may be some differences based on which welding process is being used and if it is performed manually or automatically.
Welding is a surprisingly complex process involving a wide range of variables that influence weld quality. When welds are defective, one of more variables not adjusted or controlled properly for the application has been the cause. The Inspector needs to analyze the discontinuity, investigate how it was welded and then problem solve to determine which variable(s) were likely to have caused the problem. When welds have been identified as defective and will be repaired, the Inspector should monitor the repair process carefully.
Comprehensive weld inspection is a much more detailed process than simply marking weld inconsistencies with a marker for repair. It requires knowledge including welding processes, metallurgy and inspection methods. To ensure this is done correctly, many codes including CSA, AWS and IIW require that Welding Inspectors demonstrate their competence through qualification certification.
When qualifying a welding procedure specification (WPS) for fillet welds per Clause 4 of AWS D1.1 Structural Welding Code you must only perform a single test. You must do macroetches in three locations as shown below.
Macroetch locations for welding procedure qualification of fillet welds.
It is assumed that if we can show root fusion at these three locations that our welding procedure is acceptable. Meaning it is capable of producing good welds.
However, when doing a welder performance qualification there is a difference in the test. The welder must stop at the middle of the test coupon, do a restart and finish out the weld. Instead of three macroetches only two are taken, each 2 inches from each end. The middle section of the coupon is then subjected to the fillet break test. The image below shows the location of the macroetches and the stop/restart.
A stop and restart near the middle of the weld is a requirement for the welder performance qualification test.
Spot Welding Acceptance Criteria
A macro etch gives us a clear indication of whether or not we achieved root fusion. Root fusion shall be achieved per AWS D1.1 in order for the welder to be qualified. This should be shown on both macroetches as seen below.
Macroetches show whether or not root fusion was achieve. it also provides details such as depth of root penetration, penetration depth onto the side walls, area affected by the heat (HAZ), size of legs, effective throat and more.
It is not necessary to use a camera that provides magnification as the one used for the images above, but it can help if achieving root fusion was in question. The red lines are added to clearly show the root of the joint (intersection of the red lines). Macroetches provide a lot of useful information, but they only provide information on single location of the weld. Move half an inch and the results may be different.
Since these macroetches are taken two inches from the start and two inches from the end we get reasonable assurance that the welder was able to maintain adequate travel speed, transverse and travel angles, steady contact tip to work distance and good technique.
Stud Welding Acceptance Criteria
The fillet break test is done to assure that there is fusion to the root at the start and stop done in the middle of the weld as well as throughout the weld rather than a single location. This can be a problem for some welders. The fillet break test is done by placing the root of the weld under tension. This can be done on a press or by placing the test coupon on a vice and delivering a few hammer blows as seen below.
When performing this test make sure that there are not tack welds on the other side of joint. Tack welds must always be placed near the ends of the coupon.
Asme Visual Weld Acceptance Criteria
If the plates break off it does not mean that the test failed. This is a common misconception. Whether the pieces separate or just bend the surfaces must be examined. If you can see the root (the straight edge of the vertical piece) the welder fails.
Fillet break tests reveal whether the welder was able to achieve root fusion over a section of joint rather than a single spot.
The arrow indicates the stop and restart. Notice the jagged edge on the top piece. This is indicative that root fusion was achieved throughout the joint.
The arrow points to the root of the joint. Being able to see the edge of the top piece is indicative of lack of root fusion.
Welding Distortion Acceptance Criteria
The above image shows what lack of root fusion would look like. Notice the straight edge.
Prior to performing the macroetch and the fillet break tests the weld must pass visual inspection. Acceptance criteria are provided in Clause 4 of AWS D1.1. Similarly, other codes and standards will provide their own acceptance criteria.
Visual Acceptance Criteria
The fillet break test is more often than not failed not as a result of poor skill by the welder, but by a welding procedure that is poorly developed. If the amperage is not high enough you may not always get root fusion. Introduction to fluid mechanics 5th edition pdf.
Reference:Qualification of Welding Procedures, Welders and Welding Operators per AWS D1.1 (2017)
Welding Acceptance Criteria Examples
Do you want more information on how you can perform your own welder and welding procedure qualifications?
'Qualification of Welding Procedures, Welders and Welding Operators' is a guide developed to help you perform your own qualifications in full compliance to AWS D1.1 Structural Welding Code (Steel).
