Gerald,
Because of the effect of grain size on mechanical properties ( tensile and impact properties) of carbon steels, micro alloying elements which combine with oxygen or nitrogen to produce inclusions are usually added to restrain the grain growth - this is called ''pinning'' of grain boundary. At higher temperature, depending on the type of inclusions formed, these inclusions decompose thereby losing their grain boundary pinning ability.
There is specific temperature range at which this happens . The level of grain growth depend on time at this temperature range. The higher the carbon steel at this temperature range during solidification which is controlled by cooling rate, the more the grain growth which in turn negatively affect both tensile and impact properties. Therefore specifying the maximum inter pass temperature is to limit the amount of time at this temperature range.
Also the cooling rate depending on hardenability of the material, controls microstructure formed which also influences susceptibility to cracking. The factors that affect hydrogen cracking are - level of hydrogen, susceptible microstructure and level of restraint. Preheating reduces the cooling rate thereby improving microstructure formed and also allow more time for evolution of hydrogen from the weld.
Higher cooling rate reduces the impact property but influences the formation of less susceptible microstructure.
Slower cooling rate increases impact property but favours formation of more susceptible microstructure. Hence the intent of upper and lower preheat/inter pass temperature is to limit welding within range of cooling rate to ensure that microstructure which determines mechanical properties and soundness of weld qualified during PQR qualification is maintained to meet the project requirements.
Note:
1. Preheat temperature controls the cooling rate of the first weld.
2. Inter pass temperature ( the temperature of the material just before the commencement of the subsequent weld usually measured at the bevel edge) controls the cooling rate of subsequent weld. It's not the maximum temperature attained during welding.
On Friday, 11 May 2018 05:36:46 WAT, Kannayeram Gnanapandithan <kgpandithan@gmail.com> wrote:
Why don't get interpretation from aws which will be very useful for all members. Why this code is different in somany aspects on essential variable from ASME, for example, position is essential for PQR, Product form etc.
On Thu, 10 May 2018 5:29 pm 'wilfred kojo' via Materials & Welding, <materials-welding@googlegroups.com> wrote:
--Hi,It is true that inter pass temperature is measured after the pass (just before the commencement of subsequent pass) but why should it be controlled?It's not measured because of previous pass but subsequent pass because it serves as preheat for subsequent pass which also control the cooling rate and mechanical properties.On Thursday, 10 May 2018 10:36:16 WAT, 'wilfred kojo' via Materials & Welding <materials-welding@googlegroups.com> wrote:Hello Experts,I think something is wrong here, we are interpreting code without understanding the background and the intent of clause requirements.What is the difference between preheat temperature and inter pass temperature?The preheat temperature is for first pass for the commencement of welding.The inter pass temperature is the preheat temperature of the subsequent weld that why once PQR is qualified, the minimum preheat temperature also become the minimum inter pass temperature. In the same way maximum inter pass temperature also become the maximum preheat temperature qualified.If the inter pass temperature of 110 deg. did not affect the mechanical properties of subsequent weld negatively why will the preheat of 110 deg.Note minimum preheat temperature is essential variable while the maximum inter pass/preheat temperature is supplementary essential (only applicable when impact property is essential).The maximum inter pass temperature is applicable because at higher inter pass or preheat temperature cooling rate will be low which lead to grain growth (the same reason why heat input is also supplementary essential variable). The higher the grain size the lower the impact property.On Thursday, 10 May 2018 05:41:03 WAT, Ramesh <rameshkannandevaraj@gmail.com> wrote:Hi
Practically your PQR was not preheated. It's room temperature and it was recorded as 30 Deg C. but your WPS indicates preheat 110 Deg C !!! how is this acceptable when code does not allow to increase more than 56 Deg C from your PQR,
i.e. 30+56 is maximum allowed in WPS. So u have to restrict up to 86 Deg C NOT 110 Deg C
in case of interpass , your PQR recorded 173 Deg C, so 173+56 is maximum allowed in WPS i.e. 229 Deg C
In your case , preheat temperature is mentioned wrong i.e. 110 Deg C instead of 86 Deg C which is not acceptable. IP temperature mentioned / restricted 220 Deg C which is OK.
Thanks and Regards
Rameshkannan Devaraj
M.Tech in Welding Engineering
"What you have learned is a mere handful; What you haven't learned is the size of the world – Avvaiyar"
--On Thu, May 10, 2018 at 9:13 AM 'Rajnish Dixit' via Materials & Welding <materials-welding@googlegroups.com> wrote:Hi,Please check respective WPS and refer the recorded maximum inter pass temperature. The condition is applicable if there will be increment of more than 56 deg Celsius in recorded/qualified inter pass temperature.--Thanks,
Warm Regards,
Rajnish Dixit
Lead - QA-QC/QMS/Welding/NDT/Inspection
+91 972 4200 357
On Wed, May 9, 2018 at 18:11, harish katesia<harish.katesia@gmail.com> wrote:hello,The subject under discussion is Max. preheat..It is 173 °c( Preheat/Interpass Temperature7) An increase of more than 100°F [56°C] in the maximum preheat or interpass temperature qualified )Max. preheat = Interpass temperature...So anything between 30°c to 173°c is ok.--On Tue 8 May, 2018, 7:27 PM 'james gerald' via Materials & Welding, <materials-welding@googlegroups.com> wrote:--Moreover though maximum Interpass temperature controls the Austenitic Grain size and thus the Charpy values and Preheat controls the Temperature between 800-500 degC , but under ideal conditions using ASME Materials.But below also have a effectChemical comp, method of maufacture, product form, Hardness, strength, Heat Treatment, microstructure, uniformity, homogenity, Grain size etc.. which are mostly controlled in the ASME Materials since its low temperature properties are controlled whereas material used in AWS D 1.1 have a effect.Thanks & Regards
J.Gerald Jayakumar
0091-9344954677
On Tuesday, May 8, 2018, 1:10:17 PM GMT+4, Kannayeram Gnanapandithan <kgpandithan@gmail.com> wrote:In ASME, % carbon is restricted to max 0.35%, where as in AWS it is more and properties will be deteriorated if use higher Pre Heat than qualified. That is why it is restricted.--THANKS & BEST REGARDS,KG.PANDITHAN, BE, IWE, AWS-CWI, CSWIP 3.1,CONSULTANT-WELDING & QUALITYMobile no: +919940739349On Tue, May 8, 2018 at 1:03 PM, George Dilintas <dilintas@gmail.com> wrote:in ASME IX also the increase is only for the interpass temperature and the decrease for the preheat.The increase of interpass is a supplementary essential variablethe decrease of preheat is an essential one--2018-05-02 6:54 GMT+03:00 N VENKATESWARA PRASAD <weldengr.velosi@gmail.com>:Dear Experts,--We have received a comment on Increase in Preheat temperature greater than 56°C is essential variable as per AWS D.1 from our client.
My interpretation is the increase in 56°C is applicable only for inter pass temperature. The details are as given below.
PQR Test coupon thickness : 30 mm
PQR Preheat temperature : 30°C
PQR Inter pass temperature : 173°C
Job Thickness : > 38 mm to 65 mm ( As per Table 3.3)
Preheat temperature : 110°C
Inter pass temperature : 220°C
MDMT : -40 °C
As per AWS D 1.1 Table 4.6 Point no. 7 an increase more than 56°C in max preheat or interpass temperature is essential variable for CVN testing applications.
Based on the above clause, the client is insisting that the Qualified Range for thickness in WPS shall be restricted to 38 mm as the preheat is increased from 30°C to 110°C which is not acceptable .
Please provide me if you any interpretation on it from AWS.
RegardsPrasad
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--Dr. Georgios Dilintas,Dipl. Ing. In Aeronautic and Space EngineeringPh.D in Mechanics of Solids - Computational MechanicsA.I.S, A.N.I, IRCA Lead AuditorWelding, Stress Analysis, Corrosion, QA/QC, Failure Analysis, Risk Analysis
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