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API 510 Preparatory Class
Lesson 6
Postweld Heat Treatment
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The Post Weld Heat Treatment mandatory requirements for
and, time at temperature are based on the base metals
thickness. The Code defines thickness at a welded joint in avery specific way. As follows:
UW-40Definition of Nominal Thickness for
Butt Welds
Page 150Section VIII
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(f) The term nominal thickness as used in Tables UCS-56,
UCS-56.1, UHA-32 and UHT-56, is the thickness of the
welded joint as defined below. For pressure vessels or parts
of pressure vessels being postweld heat treated in a
furnace charge, it is thegreatest weld thicknessin any
vessel or vessel part which has not previously been
postweld heat treated..
UW-40Definition of Nominal Thickness for
Butt Welds
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(1) When the welded joint connects parts of thesame
thickness, using a full penetration butt weld, the nominal
thickness is the total depth of the weld exclusive of any
permitted weld reinforcement.
UW-40Definition of Nominal Thickness for
Butt Welds
Depth of
weld
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(5) When a welded joint connects parts of unequal
thicknesses, the nominal thickness shall be the following:
(a) the thinner of two adjacent butt-weldedparts including
head to shell connections;
UW-40 Definition of Nominal Thickness for
Butt Welds
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UCS-56 Requirements for Post Weld Heat
Treatment Page 179Section VIII
PWHT is performed to specific rules based on the thicknessof the weld to be heat treated. We will now examine those
rules.
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UCS-56 Requirements for Post-Weld Heat
Treatment
(a) Before applying the detailed requirements andexemptions in these paragraphs, satisfactory weld
procedure qualifications of the proceduresto be used shall
be performed in accordance with all the essential variablesof Section IX includingconditions ofpostweld heat
treatment or lack of postweld heat treatment.
Question: What must always be present prior to welding?
Answer: A Section IX qualified welding procedure.
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UCS-56 Requirements for Post-Weld Heat
Treatment
(d)The operation of postweld heat treatment shall be carried
out by one of the procedures given in UW- 40 in accordance
with the following requirements.
(1)The temperature of the furnace shall not exceed 800F(427C) at the time the vessel or part is placed in it.
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UCS-56 Requirements for Post-Weld Heat
Treatment
(2)Above 800F(427C), the rate of heating shall be not more
than400F/hr (222C/ hr)divided by the maximum metal
thicknessof the shell or head plate in inches, but in no
case more than 400F/hr (222C/hr). .
400F/ 2 inchesso no more than200F/hr
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Class Quiz
UCS 56
1. In the example below what is the shortest amount of time
allowed to raise the weldment to a PWHT of 1100F if the
furnace is at800Fwhen the part is inserted?
Answer:___________________________________
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Solution
1. In the example below what is the shortest amount of time
allowed to raise the weldment to a PWHT of 1100F if the
furnace is at800Fwhen the part is inserted?
1100 - 800F =300F/200F = 1-1/2 Hours
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UCS-56 Requirements for Post-Weld Heat
Treatment
Turn to Page 180of Section VIII as we will now examine the
requirements for PWHT using the tabular form of UCS-56 for
P-Number 1 base metal.
This is just one Table, there are many more based on the
materials P-Number. The others follow the same format and
once we have learned to use this one the others will be
much easier to understand. You are responsible for all of the
tables on this examination however most questions comefrom the P-Number 1 Table!
The tables cannot be interpreted with out reading the notes
that are beneath them. Lets have a look.
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UCS-56 Requirements for Post-Weld Heat
Treatment
We can gather the following from the Table for P-Number
1 materials that;
The normal holding temperature is 1100 oFfor P-No. 1.
The minimum time at holding temperature is based onthe thickness of the part.
Note 1 references alternative PWHT holding
temperatures
Note 2 determines the thickness at which PWHT ismandatory.
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Table UCS-56 P-Number 1
There are three cases of thickness listed in the table P-1.
Up to 2 inches (51 mm) the PWHT is held for 1 hour per
inch (25 mm) of thickness with 15 minutes minimum in all
cases. The 15 minute minimum applies to cases where;
1. The vessel is in lethalservice and requires PWHT in all
thicknesses.
2. The vessel is being heat treated voluntarily to prevent a
service induced problem such as cracking i.e. Amine
service.
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UCS-56 Requirements for Post-Weld Heat
Treatment
Note (2) Postweld heat treatment is mandatory under the
following conditions:
(a) for welded joints over 1-1/2 in. (38 mm) nominal
thickness; ( So 1- 9/16 (40 mm) and greater would
require PWHT)
(b) for welded joints over 1-1/4 in. (32 mm) nominal
thickness through 1-1/2 in. (38 mm) nominal thickness
unless preheat is applied at a minimum temperature of
200F (93C)during welding;
Based on the various thicknesses up to 2 inches we have
the following graphical representation of these rules.
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UCS-56 PWHT Thicknesses Up to 2 in.
The Code sets the minimum thickness of a vessel at 1/16
(1.6 mm) in paragraph UG-16, one exception is for anUnfired Steam Boiler which has a 1/4 (6 mm) minimum.
1/16" to 15 min. 30 min.
45 min.
1 - 1 hour.
> 1- 1:15 min. No Preheat
1- 1:30 min. No Preheat
1- 1:45 min.
2 2 hours.
Exceeds 1- up to
1- if no preheat
applied.
1/16" to Lethal
or Service reason
> 1- Thickness
Reason For PWHT 1/16 to 2 inches
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UCS-56 PWHT Thicknesses Over 2
The second thickness range:
Over 2 in. (51 mm) to 5 in. (127 mm) the PWHT is heldfor a flat 2 hours for the first 2 inches (51 mm) of
thickness with an additional 15 minutes per inch over 2
inches. Lets look at a graphic of this thickness range.
First 2 in. (51 mm) 2 hours
OVER 2 in. to 5 in.
(51 mm to 127 mm)
Additional 1 in. (25 mm) 15
min. - Total 2:15 min.
3 in (75 mm)
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UCS-56 PWHT Thicknesses Over 5 in.
The third thickness range:
Over 5 in. (127 mm) the PWHT is held for a flat 2 hoursfor the first 2 inches (51 mm) of thickness with an
additional 15 minutes per inch over 2 inches. For P-
Number 1 there is no change from the previous example.
This third range does changes for some of the other P-Numbers. Look at the P-Number 4 Table for example;
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Class Quiz
UCS-56 PWHT
1. What is the minimum PWHT time and the minimum
holding temperature for a P-Number 5A material that is 3
(inches) thick (75 mm)?
2. What is the PWHT normal holding temperature and timefor a P-Number 3 material that is 3 (76 mm) thick?
3. What is the required time at the minimum holding
temperature for a P-Number 1 Gr.1 weld that has a nominalgoverning thickness of 1-1/4 ? The vessel will not be in a
lethal service.
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Solution
1. 3 hours at 1250 oF (677 oC).
2. 1100 oF (593oC) 2 hours -15 minutes?
3. There is no mandatory heat treatment in this thickness, itmust exceed 1-1/4
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UCS-56.1
Alternative Post-Weld Heat Treatment
Requirements for Carbon and Low AlloySteels Page 187
By Note 1 of Table UCS 56 for P-Number 1 materials, it is
possible to PWHT at a temperature lower than that given in
Table UCS-56. It involves heat treating for a longer periods
of time, based on the amount of reduction in temperature
below the stated minimum in Table UCS-56.
The following Table UCS-56.1 outlines these rules.
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Table UCS-56.1
Alternative Post-Weld Heat Treatment
Before going any further it must be cautioned that you
cannot usethis alternate unless you have been referred to
it by a note in one of the material tables. In our example we
are using Note 1 referenced by P-Number 1.From the table we find that there are three columns.
The left column lists the decrease in temperature below
that given on the appropriate table in UCS-56 based on
material.The center lists the Minimum Holding Time at a decreased
temperature and;
The third lists references to notes below the table.
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Table UCS-56.1
Alternative Post-Weld Heat Treatment
Reading Note 1 found below the chart and referenced up in
the Minimum Holding Time column we see;
(1) Minimum holding time for 1 in. (25 mm) thickness or less.
Add 15 minutes per inch(25 mm) fro thickness greater tan 1in. (25 mm).
Reading Note 2 listed in the Notes column;
(2) These lower postweld heat treatment temperatures
permitted only for P-No. 1 Gr. 1 and 2 materials.
As regards Note 2, there is a P-No. 1 Group 3 material. So
be cautious on the exam this could be a trick question.
Note 1 is best addressed using a graphic as follows;
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UCS-56.1 Alternative PWHT
We will first examine a 50oF (28oC) drop from 1100 to1050oF.Below is the holding time from our previous 3 coupon based
on 1100oF. How long would we be required to hold it at
1050oF?
First 1 in. (25 mm) 2 hours
Lower PWHT at 1050 F
Additional 1 in. (25 mm) add
15 min. Total 2:30
3 in (75 mm)Additional 1 in. (25 mm) add
15 min.
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UCS-56.1 Alternative PWHT
Which leads to this total time, up from 2:15 min. to 2:30 min.
Now how about 100oF reduction to 1000oF?
First 1 in. (25 mm) 4 hours
Lowered PWHT at 1000 F
Additional 1 in. (25 mm) add
15 min. - Total 4:30 min.
3 in (75 mm)Additional 1 in. (25 mm)
add 15 min.
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Class Quiz
UCS-56.1
What is the total PWHT time at 950oF for a 5 inch thick P-No
1 Group 2 material?
Lowered PWHT at 950 F
5 in (125 mm)
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Solution
What is the total PWHT time at 950oF for a 5 inch thick P-No
1 Group 2 material?
First 1 in. (25 mm) 10 hours
Lowered PWHT at 950 F
Additional 1 in. (25 mm) add
15 min. - Total 11 hours.
5 in (125 mm)
Additional 1 in. (25 mm)
add 15 min.
Additional 1 in. (25 mm)
add 15 min.
Additional 1 in. (25 mm)add 15 min.
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The End I Know
You're Glad