How to write a welding procedure specification (ISO 15614-1)
One of the most important documents in regards to welding is the WPS (Welding Procedure Specification) and many people working with WPS look at them like they are just an issue, or something that only the engineers and quality control technicians have to worry about, but is it a hard thing to create and/or look at? Let’s break it down into parts.
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This article will be related to ISO 15614-1 and ISO 15609-1. I shall cover ASME IX on a future article. Let’s get started with a summary of the essential variables:
- Base Material Groups (§8.3.1)
- Base Material Thickness (Table 5 for Butt Welds, Table 6 for Fillet Welds – §126.96.36.199)
- Throat thickness is also essential for fillet welds
- Diameter, for pipes (§188.8.131.52)
- Branch Angle (for branch connection welds – §8.3.3)
- Welding Process (§8.4.1)
- Process Mechanization (§8.4.1)
- Welding Positions (§8.4.2)
- Weld Type (§8.4.3)
- Product Type (§8.4.3)
- Filler Material Classification (§8.4.4)
- Type of Current and Polarity (§8.4.7)
- Heat Input (§8.4.8) – if there are hardness and/or impact requirements
- Preheat Temperature (§8.4.9)
- Interpass Temperature (§8.4.10)
- Postheat Temperature (§8.4.11)
- Post Weld Heat treatment (§8.4.12)
Note: Take a look at §8.4.5 for process specific essential variables
Some of these informations are not related directly to ISO 15609-1, but your clients will want you to specify which project this WPS is related to and, as such, you should also include your client’s company in there.
The biggest and most relevant piece of information here is the WPS Number (in this case I’ve named it “Sample WPS (ISO)”, but you should use your own designations.
There should also be spaces where logos should go. In this specific example I have two logo spots, in which the left one should be your company’s logo, leaving the right space for your ISO 9001 certification body, or your client’s logo.
Most WPS will be backed on a PQR (Procedure Qualification Record) and this information should always be stated in the WPS. All you need to do is reference the PQR number which will allow any reader of your WPS to trace back to the original qualifying document.
The standard is also related to the PQR, in this case it is ISO 15614-1, but ISO 15609-1’s WPS format is relevant to other standards, such as ISO 15614-2/3/4/5 or even ISO 15613.
As you already know, PQRs always have approval ranges based on what was welded for the approval of that specific test. One of the main points in how to elaborate a WPS is that you may use these approval ranges to limit your specification, but in some cases you need to specify accurately what will be welded on the job. The first example for this is the product type:
This WPS is meant for piping work. Sometimes the PQR will approve welding Pipes and Plates. This means you could technically use a WPS for piping and plating / structural work. That is not the point of the welding procedure specification though, as you should specify the details so that there is no room for doubts when a welder interprets this document.
The same goes for the weld type, in which some standards will approve welding fillet welds by performing a butt weld test. Like the product type, you should specify which type of weld will be made with this WPS, and only choose a single option.
Welding processes are pretty straightforward. Just type in the processes used in the PQR / the ones to be used in production.
Base Material Details
The base material details are also quite straightforward. Keep in mind that this is WeldNote’s WPS format and as such you do not need to place the fields exactly where I’ve put them.
First of all i’ll cover the base materials themselves, which are intertwined with the groups. These groups are what define which base materials should be welded with this WPS. These group definitions can be found at ISO/TR 15608, but if you’d like to know what group a material belongs to, you should check out ISO/TR 20172 and ISO/TR 20173.
NOTE: You may notice that the base material’s grade and specification come from an american standard (ASME / ASTM). While using ISO standards, it is preferable to use ISO designations for this steel (e.g. EN 10028-7 X2CrNiMo17-12-2), but while preferable, it is not mandatory, as long as the base material groups are correct.
In this case, the thickness range is exactly the approval range limits of the PQR. While this is perfectly acceptable in regards to standards, you should restrict the limits of your WPS in order for them to be accurate with the amperage, voltage and travel speed parameters (you would not want to weld a 3 mm plate with the same parameters you would use for a 24mm plate).
The same goes for the diameter, but i’d say this one is more lenient. My suggestion would be to use a WPS for small thicknesses and small diameters, and a WPS for bigger thicknesses and bigger diameters (more, if the project has a lot of different pipe or plate sizes).
Always use a range of thicknesses and diameters though, even if they are a bit strict. You wouldn’t want an inspector to come to your shop floor and order you to stop using a 15mm WPS because the welder is welding a 15.2mm pipe (which has already happened before).
Welding positions are quite important, and you should plan for them ahead of time. The safest way to go at it is to put them all in your WPS, as it is approved by the PQR, but the best way to proceed is to define which are the positions to be used in the shop floor, which will mostly be PA, PC and PH.
Filler Material Details
Getting into the filler materials or consumables, in my experience there have been some misconceptions on some of the requirements here.
The specification and classification of the filler material should be exactly as the standards (AWS, ASME or ISO) say. This is for a 316 stainless steel and as such I’ve chosen the ER316L for both TIG and MAG processes.
NOTE: Like the base materials, in the filler materials you should preferably use ISO specifications and classifications. But once again it is not something required.
The commercial name for a given filler material is not mandatory as per ISO 15614-1, it is not an essential variable unless you are using the following processes: 111, 114, 12X, 136 and 137, so according to §8.4.5 only the “fluxed” processes require your WPS to have a filler material commercial designation in them, and even then, it’s only when impact tests are required, even if that is true for most cases.
In regards to the diameter or filler material size, according to ISO 15614-1, you can change it, so long as you take into consideration the heat input requirements and, as such, I would recommend you to have 2 to 3 options here (e.g. 2.0mm and 2.4mm for the TIG rods).
You can find the symbol for the gas designations in ISO 14175, I1 means 100% Argon and M21 means a mixture of Argon with 15 to 25% CO2. This information will probably come in your gas certificate along with the commercial name, which is something that is not at all required to put in your WPS, however, you probably want to put it, especially if you work with more than one gas supplier. Why? This way your welders will know exactly which gas tank they should pick up and avoid any problems with your client about the specified gas for the job at hand.
Flow rates are important, they make the difference between a welder being able to control the torch easily and assuring a sound weld, with no oxidation either on the root or the top side, which means this value should be thought out and based upon previous experience, for both the shielding and backing gases.
While the joint details are only intended to give some guiding lines for the welders, their importance should not be underestimated. It is very hard to define exactly the number of passes on the weld layer configuration, and as such you should have, at most, a range of the maximum number of layers to be made. The groove design should have important information such as the root gap, root face and groove angle (which could mean it’s meant for fitters and tack welders).
Preparation methods do not need a lot of details, you should just specify if this welded joint is going to be grinded, machined, etc.
The weld details are an essential variable so they are of the utmost importance, but you just need to specify whether the joint will have material, gas or no backing, or if it will be welded in a single side or both sides. You may mention the fact that it could be a single or multi layer weld, but that is not so important, as that information will already be available on the welding passes themselves.
Now we’ve reached into the most important part of the WPS. The detailed description of how each pass should be welded. This should be filled in with care, with information based upon your company’s welding experience and your certifications.
- Pass #: This is pretty straightforward as you only need to specify which layer you’re referring to. You may put a range of layers (e.g. passes 3 to 6) or if you’re referring to the remaining passes until the capping layer you may use “passes 3 to N” or “3+”.
- Process: Name the processes to be used for the specific welding pass
- Filler Material: The filler material should always be metallurgically, physically and chemically compatible with the base material and should be chosen prior to this WPS elaboration. In this case you just have to associate it to the corresponding welding process and pass.
- Diameter: The fact that the filler material diameter or size is not an essential variable gives you some freedom in what to insert here, however, as the heat input is an essential variable, you will need to take care in selecting a proper diameter. The rule of thumb is to use a small diameter for the first passes, and raise the diameter as you weld more layers. On higher layers you can raise the amperage, voltage and travel speed to gain some productivity while maintaining the heat input levels, but for that you also need a larger rod, electrode, or in the case of semi-automatic processes, a higher wire feed speed.
- Current: These parameters are the most critical to assure a sound weld. Amperage has a direct influence on the heat input and also defines how a welder will control his puddle. In case of a semi-automatic process you will not control the current directly. You will, however, be able to control the voltage and wire feed speed. You should either use previous experience to define these values, or hire an experienced consultant to help you with them.
- Voltage: If you are using a manual welding process, you cannot control the voltage, as this value is defined by the height of the welder’s hand, relative to the pipe being welded, and as such you should set a considerable range of values, regarding to your welders’ skills and usual techniques. In the case of semi-automatic welding, the voltage is a set value that will remain the same throughout the welding pass, and should be defined, like the current, with previous experience in mind, or with the expertise of an experienced consultant.
- Polarity will have a big influence on the deposit rates of your weld, but it will also influence your penetration. On ISO 15614-1 you are not allowed to change this parameter from the PQR, although you should always use higher penetration polarities for the root passes (DC- for TIG and DC+ for the other processes) and higher deposit rates polarities for some of the filling passes and or capping passes (DC+ for TIG is not really recommended in this case, but DC- in other processes is doable)
- Wire Feed Speed: Only applicable to semi-automatic and automatic processes, like the voltage and the current, it is something that should be based on previous experience or from an experienced consultant’s knowledge. Selecting a wire feed speed from your PQR’s parameters is also a viable option.
- Travel Speed should always be accounted to an acceptable heat input, however, if you are using manual welding, you can have a wider range of values, as you will have to take into account that each welder has different techniques and may reach sound welds with very different travel speeds.
- Heat Input is a critical value that has to be calculated. All the other parameters have to boil down to bring this to acceptable values, as it will be a way to determine if a weld is sound or not. Calculate this with the following formula
There are some additional details that are, nonetheless, relevant to making WPS according to ISO 15609-1 and ISO 15614-1. Although less important, they should be mentioned. The most important here would be the torch angle and number of electrodes. The torch angle will directly influence the morphology of the weld bead.
The oscillation is mostly used for machine welding, but you can define the limits of the weaving that a welder should restrict himself to. Details of back gouging are only relevant if you are performing welding from both sides, and could have some information of the electrode used, how much thickness and width to be gouged, etc.
Pulse weld details is similar, but only if you are welding with a pulsed transfer mode / current type, and should have the current minimum and maximum peaks, with the period between pulses, etc. Stick-out has an effect on the heat developed on the electrode, causing it to eventually raise or decrease the deposit rate.
The thermal parameters are usually pretty straightforward, even if they are of the utmost importance (they are straightforward from a WPS standpoint, but if you have to specify a welding procedure before it is qualified it becomes a serious design issue, as it will take into account your company’s resources and shop areas, as well as material types).
The preheat temperature is stated on your PQR, and according to ISO 15614-1 it should not be lower than what is stated there. As such you could opt to go with the minimum value or if you find there is an extra need for preheating, you may do so (e.g. higher thickness requiring a higher temperature).
The interpass temperature is especially important when you’re dealing with austenitic stainless steels, but should not be disregarded for any other material (I would recommend about 150 for austenitic, and 300 for carbon steels).
Post weld heat treatment is perhaps one of the most important aspects of your welding, if it is verified to be necessary. In high thicknesses it is mandatory in pretty much every welding code, in Cr-Mo steels as well, in order to prevent the forming of cracks due to three dimensional cooling rates or the high temperability of these Cr-Mo steels.
The time field is the duration of the heat treatment at the highest temperature, usually 1 to 2 hours, depending on the steel and application. The method is dependant on your company’s resources. If you have to perform work outdoors you won’t have an oven readily available and as such you may have to use an oxypropane torch. The heating and cooling rate should be around 100ºC / h, especially for the cooling rate, which should not be too high.
Revisions and Approval
Finally, the revisions should be numbered (or lettered), with the signatures of the persons responsible for writing the wps, verifying it and approving it, to be provided to your client to sign and approve it themselves.
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