Nick Verberkmoes

Nick Verberkmoes

After more than twelve years of experience in the oil and gas industry, Nick decided to continue as an independent entrepreneur. In this way he can use his knowledge and experience in a broad way.

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Heat input – part 2

Heat input ranges for your WPS based on codes

This is part two about heat input. This article is about dealing with the variety of welding codes and the heat input range on your WPS. It can be read without part one, however terms such as WPS, WPQ and heat input are defined in part one. So, it might be worth having a glance at part one, even if it is to refresh your memory.

Codes; how to make them work 

Within my welding career I have worked with many design codes, welding standards and specifications, from here on all will be referred to as codes. Codes are created based on engineering knowledge, judgement and experience, and in general are there to design and build safe products. Since engineering perspectives differ throughout the world, codes may vary quite a bit. 

Zooming in on heat input ranges; some codes state ± 10% up to ±25%, others solely allow the maximum qualified or accept a range on amperes, volts and travel speed. A wide variety can be found and we must adhere to the applicable code, right? My answer is a firm yes. This immediately shows one key element for creating the heat input range on a WPS; you must understand the requirements of the applicable code. 

A question I often get from contractors is “What qualified position gives us the largest range?”. While their real question is “How many WPQ’s must we make?”. Followed by “Can we keep the costs as low as possible?”. My answer to these questions is that this depends on what is needed in production. People tend to forget that the WPS is for the production and a WPQ provides support to the WPS. You may try to develop a WPQ to cover everything in production. To create the largest ranges on parameters such as multiple weld positions, pipe and plate, thin and thick material. These kind of WPQ’s often lead to unusable parameters for production welding. 

Irrespective of any code, it is recommended to qualify at least the horizontal and vertical up positions. When comparing these weld positions the horizontal requires in most cases the fastest and the vertical up the lowest welding speeds, leading to the lowest and highest heat inputs. This interacts with the behaviour of structural (C-Mn) metals, where in general the low heat input may result in hardened material structures and the high heat input in possibly lower impact properties.  

Some examples

Let’s discuss the examples given in the table (1) below. Considering heat input as the only varying parameter, one code applies ±25% (version A) and the other code requires the highest qualified heat input (version B).

Example

Welding positions

Weld layers

Heat input range in WPQ [kJ/mm]

Version A

[kJ/mm]

Version B

[kJ/mm]

1

Single 

2G / PC

Single

1.25

0.94 – 1.56

1.25

2

Single

2G / PC

Multiple

1.00 – 1.50

0.75 – 1.88

1.50

3

Multiple

2G / PC

Single

0.90

0.68 – 1.13

1.80

3Gup / PF

1.80

1.35 – 2.25

4

Multiple

2G / PC

Multiple

0.70 – 1.25

0.53 – 1.56

1.90

3Gup / PF

1.60 – 1.90

1.20 – 2.38

Table 1. Examples for heat input discussion

Despite my advice in part one on using minimum and maximum heat input values, for educational purposes I use single heat input values in some of the examples. 

Example 1: For version A you calculate the heat input by single value 1.25 kJ/mm ± 25%. For version B the highest qualified value of 1.25 kJ/mm shall be stated on the WPS.

Example 2: Now a range with a minimum 1.00 kJ/mm and maximum 1.50 kJ/mm is given. In version A the heat input for the WPS is calculated by the lowest value -25% = 0.75 kJ/mm and the highest value +25% = 1.88 kJ/mm. In version B the highest qualified range applies so the maximum heat input on your WPS shall be 1.50 kJ/mm.

Example 3: Two single values, 0.90 kJ/mm and 1.80 kJ/mm, are given. In version A applying the ±25% would lead to two heat input ranges which do not overlap. Then the question would be, are we allowed to combine these ranges? Well yes, since you have WPQ’s qualified on the low and high heat input, so any value in the total range can be considered acceptable. This would give us a range of 0.68 kJ/mm up to 2.25 kJ/mm. However, we could question if it is necessary to create a large range like this for production. This discussion should be held between contractor and client welding specialists. 

For version B the maximum qualified allows the 1.80 kJ/mm as max heat input on your WPS.

Example 4: The approach is similar as in example 3. Although I would fully understand when it is requested to keep the ranges of the WPS as per the WPQ, being 0.70 – 1.90 kJ/mm. Welding within a range of 0.70 – 1.90 kJ/mm can be achieved by most welders. 

Concluding

Conclusion of the above examples is quite simple; understand and adhere to the applicable codes. Combining of heat input values per welding position can be allowed, provided the WPQ’s meet the code and project requirements. Discussions on heat input ranges are always possible and should reflect the practical world as well. Keep in mind that welders must be able  to work according the given ranges on your WPS!

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