PWHT - [Part 1] HOW TO READ A PWHT CHART

Due to the advancement of metallurgy a lot many different materials are now used in mechanical construction industry. In this a few are heat treated to improve mechanical properties.

Heat treatment is done because of two reasons 1 ) Process requirement  2) Material Requirement

Heat treatment can be done in many ways. Furnace heating, Oil fired heating, Electric resistance heating etc.

There are different type of heat treatments like normalizing, annealing etc.

Here we are going to discuss about the graph which is generated in electric resistance heating for PWHT (Post Weld Heat Treatment).

Reading a PWHT chart is what is going to be discussed here....

Below is the graph of a pwht process. Lets start reading it.....(Here i am discussing on the understanding that you are a little bit familiar with the process....)

In this chart,

X-axis represents temperature and Y-axis represents Time.

In X-axis each line to line distance represents 10Deg C (See it is written as X 100°C, that means in X axis, 2 means 2 X 100°C= 200°C, 3 means 3 X 100°C= 300°C etc.)

In Y-axis each line to line distance represents half an hour. That means the time between 2 and 3 represents one hour.

These are the basics.

How to read the graph?

The graph contains several lines, AB, BC, CD, AE

1 ) AB is RATE OF HEATING curve

2) BC is SOAKING TEMPERATURE

3) CD is RATE OF COOLING curve

4) AE is ROOM TEMPERATURE curve

When you check the WPS you can see the required values for each of these.

1) AB: Rate of heating: In WPS it is mentioned as a maximum value. In the graph the rate of heating is the slope of the curve AB. So the value of the slope should not be more than the value in WPS.

How to calculate the slope:
                            Slope = kl/lm
Now in the graph given above, for AB line, kl=80DegC and lm=1/2 hr

slope = 80 / (1/2) = 160DegC/hr ; So in this graph the rate of heating is 160DegC/hr.

2) BC: Soaking Temperature: In WPS it is mentioned as a single value with some plus/minus allowance along with time it is to be steady(mentioned as minimum)

Here in this graph it is 720Deg (Reading in X-axis) and for approximate 2hr and 10minute(this is an approximation-Reading in Y-axis )


3) CD: Rate of cooling: In WPS it is mentioned as a maximum value. In the graph the rate of heating is the slope of the curve CD. So the value of the slope should not be more than the value in WPS.

How to calculate the slope:
                            Slope = qr/pq
Now in the graph given above, for CD line, qr=130DegC and pq=45 Min(this is taken as an approximation from the graph.)

slope = 130 / 45 = 2.89DegC/Min ; So in this graph the rate of heating is 173DegC/hr.


4) AE: Room Temperature: This is the room temperature, here it is 40 deg C.

Loading Temperature: Loading temperature is the temperature at which controlled heating (maintaining the rate of heating in the WPS) starts, up to which there is no need for controlled heating.

Unloading Temperature: Unloading temperature is the temperature up to which controlled cooling (maintaining the rate of cooling in the WPS) occurs, below which there is no need for controlled cooling.Normally the power connection is disconnected below the temperature, keeping the insulation. After getting cooled to the atmospheric temperature the insulation is removed.

NB: Please check the speed of the chart movement in the PWHT machine, Normally it is 25mm/hr.

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