Time at Temperature, Normalizing

[weldtek]

How critical is time at temperature when normalizing carbon steel?
Specifically, if the mill supplies green material with test coupons normalized for 1 hour/ inch, and the fabricator normalizes the part for 1/2 hr / inch, is there reason to be concerned that the test coupons don't properly represent the part?

 

[metengr]

weldtek;
What is the section thickness for the test coupon supplied by the mill? What section thickness is the fabricator working with for normalizing?

 

[weldtek]

They're both 1.75". The mill supplied green material to the head manufacturer who normalized the head at 1/2 hr / inch.
However, the mill had normalized the test coupons for that material at 1 hr / inch.

 

[metengr]

weldtek;
Normally, the 1 hour/inch is a conservative value and accepted norm to allow an object to reach a uniform temperature profile and complete through-thickness austenitization.

Did you review the recorded austenitizing temperature of the fabricator and compare to the temperature recorded by the mill? It may be that the fabricator austenitized at a higher temperature.

If the austenitizing temperature of the coupon was within 100 deg F of the fabricators austenitizing temperature, I would be concerned about an inadequate normalization heat treatment. I would perform bulk hardness testing at several locations and compare to the coupon hardness, just to be sure.

 

[ulyssess]

How do you define "time at temperature"? Where do you measure the temperature?
If you can assure that the whole section of your piece has reached the target temperature, a carbon steel does not need any more time at temperature (esp. for C-contents < 0,30%). Holding on temperature is not necessary, but does not have a negative effect neither.
Rules like 1 hour/inch are often chosen to assure an equalisation of the temperature over the whole section when temperature is measured at the surface. These rules are very approximative as the time for equalization is strongly dependant on the heating rate, heating capacity of the furnace, position of the burners, form of the product (plate, shell, head,..) ...

 

[strider6]

as ulysses said the time @ temperature is just a matter to allow all the material to reach the temperature. When the material reach the temperature then is not necessary to hold it.

S

Corrosion Prevention & Corrosion Control

 

[Helicopterjunky]

Weldtek...Easy way to tell is to take part and drill hole to center of section. Insert thermocouple into center of section and a thermocouple to outside surface of part. Load the part with equivalent furnace, furnace load, same loading, # parts, # pounds, # trays or fixtures as manufacturer. Record surface and core temperature and well as furnace temperature. Plot all three temps.versus time and this should give you a pretty good idea how long it takes for the core to reach austenitizing temperature (say within 10F of furnace setpoint) versus how long the surface and furnace thermocouple need to reach reach temp. Goal is to have center of largest section at austenitizing temperature before commencing still air cooling.