Heat Input

Date 10-13-2009 03:44

That is open to debate.

Some companies want the PQR to run with consistent heat input for each weld bead. Two PQRs are required, one PQR for the high and a second PQR for the low heat inputs. They also recommend that mechanized "tractors" be used to ensure consistent travel speeds to aid in ensuring consistent heat inputs. The intent is to bracket the heat input so that anything in between can be used assuming both the high and low heat inputs pass all the requisite tests.

Some smaller fabricators don’t have the mechanization necessary to run a tightly controlled PQR. In those cases I record the heat input for every pass and try to minimize the variation in travel, volts, and current. I then use statistical analysis to determine the median heat input and the standard deviation. If the SD is less than 10%, I call it a day. Two PQRs are still the norm if it is required by the applicable welding standard, one with high heat input and one with low heat input. The median values are reported for the high and low for the purpose of the documentation. All of the data, including the actual heat input of each bead, is made part of the test record and the method used to calculate the median and standard deviation is noted. I haven’t had any of the PQRs rejected.

There are some welding standards where the maximum heat input is specified and the ach weld bead is held to something less than the maximum permitted by the standard. I still record the variables for each weld bead. I completed a WPS recently that consisted of about 80 passes. That’s a lot of data.

Best regards - Al

By vz776j Date 10-13-2009 13:25

Thanks for responding...I will have more questions later.

By trapdoor (**)

Date 10-14-2009 07:16

Al
What is your take on inspecting (AWS D1.1) welds requiring notch toughness? Does the inspector need to monitor heat input and if so how frequent?

By 3.2 Inspector (***)

Date 10-14-2009 08:12

Please note that I am not Al, nor do I have his expertise.....but
When you say "inspecting" do you mean when welding procedure tests or production welding?

3.2

By trapdoor (**)

Date 10-15-2009 02:13

Sorry for not being clear I meant when inspecting production welds.

By 3.2 Inspector (***)

Date 10-15-2009 05:13

Even though I did not become Al overnight - I dont even have the mentioned code.......

Heat input is very rarely meassured during production welding, on temper bead welding perhaps, but I even doubt that.
When you qualify your WPS you monitor and record parameters such as volts, amps and travel speed. These parameters are then put into a relative simple formula in order to calculate your heat input.
Later when you are back in the office with all the calculations and meassurements, you write your wps based on the values you recorded during weldinging of the coupon.
Depending on code you are allowed to write an increase/decrease, within a certain percentage of the recorded heat input.
This often only applys, when charpy and hardness is of concern. Now you have a qualified WPS, assuming the coupon passed the required destructive and non-destructive tests.

As long as the welders follow the qualified WPS the completed weld will have known charpy and hardness values.

I sometimes perform a "production examination" where I more or less monitor and record the above said parameters and I often find the WPS to be poorly written.

This is meant as a general rule and it maybe don't apply in your case :)

3.2

By 803056

Date 10-15-2009 21:15 Edited 10-16-2009 03:08

The inspector is tasked with monitoring the welder during production to ensure the welder is working within the parameters of the WPS, including any heat input limitations. The parameters were established when the test piece was welded and the test results verified the process produced the required mechanical properties, including the required notch toughness. The welder's parameters will vary as the test plate is welded unless the process is mechanized. For that reason, I use statistical analysis to derive the median values for voltage, amperage, travel speed, wire feed speed, and heat input.

The welder should be instructed how he is to weld and any constrains imposed by the need to limit the heat input while welding. Few welders appreciate the need to limit heat input unless they have received some instruction about heat input and its affect on the mechanical properties of the weld and heat affected zone. They also need instruction on how to monitor and control heat input.

The inspector likewise needs to understand the importance of limiting heat input and how to monitor it during welding operations. While I don't totally disagree with 3.2's position regarding the damage done if the welder exceeds the heat input limitations on one or two weld beads in a groove that requires many passes to complete, but the code is what it is and the values established by the PQR/WPS apply to each weld bead.

Codes define the limits for heat input and each code or welding standard addresses it a little differently, but the desired outcome is the same. The primary goal is to limit the heat input to maintain the mechanical properties and to limit grain size to enhance notch toughness or to minimize the degradation of the properties in the HAZ or weld. Some welding standards simply state the maximum heat input permitted. Others have the fabricator determine the maximum heat input by "trial and error", i.e., a procedure is qualified and if it passes, those parameters used are the basis of establishing the ranges of the welding parameters.

A properly written WPS can provide the welder with much needed information if it is presented in a manner that is easily understood. I find that a graphical presentation is the easiest to decipher and provides a relatively easy means of monitoring the welding parameters so some predetermined limit on heat input is not exceeded.

The trick is to balance the voltage, amperage (wire feed speed), and travel speed. For the semi-automatic welding processes the wire feed speed is constant once it is dialed in. Likewise, the voltage is a constant (hence the term constant potential power supply). Amperage will vary if the electrode extension isn’t defined, so define it! The only real variable is the travel speed. The graphic presentation can use the travel speed for the vertical axis and amperage or wire feed speed as the horizontal axis. Voltage can be presented as several “horizontal” plot lines, each with a specific voltage value.

Processes that use constant current power supplies can be presented in a similar manner. The attached sketch may clarify the manner in which I present the data.

You can also present the data using a “Triangular” presentation where voltage, amperage, and travel speed are plotted to create a “window”, but it takes time to construct and the axis are not linear.

Tables can be difficult to use. The simple graphic presentation is what I find to be easiest to construct and easiest for both the welder and inspector to use.

It isn't rocket science. If it was, I would have to find a different line of work.

Best regards – Al

Attachment: HeatInputGraphrotated13-July-09Model1.pdf (89k)