Normalizing vs 'Quench and Tempering' 3

[Aife]

Would quenching and tempering have significate impact on the properties of steel (A694) versus normalizing?

I couldn't find A694 in Section 2A of ASME. It is a Carbon and Alloy Steel for high-Pressure Service.

Any help would be greatly appreciated.

 

[bmoorthy]

A 694 is not permitted in Pressure vessels hence not a part of Section II A

You will find it in ASTM.

Why do you have such a query regarding Q T and Normalizing?

Q & T is expected to have better Impact properties at the same time have higher tensile strength.

The Micro Structure of Q & T and Normalizing are significantly different.

Beware of A 694 while ordering. Unless you specify the limits in chemistry, you may have one heat of A 694 fully confirming and meeting the chemical properties of Low Carbon steel and another heat or Lot could have 1% Cr and 0.5% Mo and yet certified as A 694.

Also some of the metallurgists consider A 694 are material whose properties are enhanced by Heat treatment hence they may consider the material as UHT material evben if they permit you to use the material in Pressure vessel.

You have to be very careful while specifying the fabrication heat treatment temperature as it might affect the physical property.

It is advisable to inform the supplier of the A 694 (if you are going to heat treat the material during fabrication) that the material is likely to be heat treated during fabrication. If you expect the physical property of the material to be preserved after Fabrication heat treatment then it is advisable to ask for Simulation testing.

Consult your AI.

 

[metengr]

Yes. For most pressure vessel steels, a liquid quench and temper heat treatment assures a rather consistent, through-thickness microstructure, which enhances fracture toughness properties. In comparison, a normalization heat treatment may only involve rapid cooling in air that would increase strength over annealed material, but not necessarily result in a through-thickness microstructure with improved fracture toughness properties as a quench and temper heat treatment.

 

[Aife]

bmoorthy:
If it isn't to be used in Pressure Vessels, why is it considered good for high pressure service?

 

[metengr]

Aife;
I can give you an "inside" technical opinion of evaluating materials for ASME B&PV Code applications, which would apply to other Codes and Standards committees. The endorsement of a particular material for use in boiler and pressure vessels requires extensive testing of numerous heats to determine minimum mechanical properties, weldability and ease of fabrication pertaining to safe use in boiler and pressure vessel service.

Certain steels even though they have been endorsed under ASTM will not be endorsed by ASME B&PV Code. There are various reasons for this; the limited use of a steel at low and/or elevated temperature service, weldability and fabrication concerns related to the specified chemical composition and melting practices endorsed by ASTM. The use of steel grades under ASTM A 694 are considered appropriate for applications in gas transmission piping systems (ASME B31.8) because the committee responsible for this Code felt these steel grades meet specific strength requirements for safe service.

 

[bmoorthy]

You will also notice the manufacturer of A 694 Forging has wide range of chemistry (very limited restrictions).
Weldability is a specific concern as you may land up with actually receiving low alloy steel material meeting typical chemistry of P4 or P5 (Sec Ix P4 and P5) grades. Where as A 694 is classified as S number 1.

I was in a fix once when we used the unlisted material clause (Using UG 10 if i recollect)and used A 694 in a pressuer Vessel. The allowable stresses were taken on the basis of Appendix of ASME Sec VIII(where as our french design engineer was radical and wanted to use allowable stresses given in 31.3!!!). When we got the material from 2 different suppliers, one italian supplier gave Low Carbon steel material (Flange and another italian Supplier gave for SR forging) gave Low alloy material. The weld procedures were to be qualified again (That was due to PWHT restriction on a Q & T material).

We then decided to not to ever go for A 694 for PV and we decided to not reap advantage of high yield of A 694 material.

The minimal advantage gained in reduction in thickness by choosing A 694 is not worth it for PV (Especially when your user is very conservative).

 

[davefitz]

The selection of quench + temper vs air cooled N+T is usually related to the component's thickness. The material properties will be related to the rate of cooling ( see the alloy's TTT curves), and the component's rate of cooling is associated with (a) the heat transfer coefficient at the outer surface and (b) the componetns thickness.

A thin component will cool much faster in air than a thicker component; for a fixed heat transfer coeficient at the outer surface, the rate of change of midwall metal temp will vary by the square of the wall thickness. A 1/2" thick plate will cool 16 times faster than a 2" thick plate. If the TTT curves require a cool rate in excess of -9 F/min ( for ecample, to avoid ferrite formation in P91), then air cooling might only be effective up to a component thickness up to perhaps 3" ( to be confirmed) above this thickness, you should quench to be assured that the mid wall metal is cooled at the required rate per TTT curves.