The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Can Galvanized Sheets be Annealed?

We have regular source of defective full hard galvanized sheets.
My question-Can be Galvanized sheets be Annealed? if yes , how ?

Vinay Didwania
product designer - India
July 19, 2008

---

First of two simultaneous responses --

Yes.
By heating in a controlled atmosphere, controlled temperature oven.

Surplus zinc melts off, and the full depth of coating becomes Zn/Fe alloy.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

July 22, 2008

---

Second of two simultaneous responses --

The lowest possible annealing temperature is well above the melting point of zinc, so annealing would destroy the galvanizing.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
July 23, 2008

---

July 23, 2008

The galv isn't destroyed at its melting temperature of 419C.
Pure zinc in the coating starts to alloy with Fe and form the various alloys on the surface, which will then withstand temps to about 500C under controlled atmosphere conditions.

There are furnaces treating fabricated steelwork (post galvanizing) in preparation for powdercoating, in order to smooth the coating, operating daily in Europe processing 10's of tonnes a day.

But the heating cycle, rate of temp increase, dwell at temps and oxidation state of the flame (natural gas) are all critical.

Its not easy, but can and is done.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

---

July 24, 2008

Sirs:
I have no first-hand experience with this issue. I think Geoff's answer is more correct. I think the proper temperature would be between 850 F and 880 F and I do not know if a controlled atmosphere would be required or not. The easy thing to do is to take some small test pieces and put them into an oven at say 875 F for five minutes and see what happens. It may take a higher temperature or longer time to get the required results.
Regards, Dr. Thomas H. Cook, Hot Springs, South Dakota, USA

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

---

Vinay wants to anneal. Annealing temperature will be around 900 C. The question is whether annealing will damage the galvanizing, not what is the highest temperature that will not damage it. If he wants to anneal, he has to use the necessary annealing temperature.

A temperature of 850-880 F is far below annealing, and is in fact in the middle of the embrittling temperature range for a simple stress-relieve. (The fact that steel is not embrittled by the process of galvanizing at that temperature is because it is at that temperature for only a few minutes).

If Vinay were to change his requirement from annealing to stress-relieving, he would then need only 600 °C instead of 900 C. This is still well in excess of 850-880 F. The question becomes whether the necessary stress-relieving temperature of 600 °C will damage the galvanizing.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
July 27, 2008

---

I wonder what exactly was meant by "annealing" in this case.

If annealing the steel is the requirement, then what Bill says is right.

But if the process is to turn galvanized steel into the coating called "Galvaneal" (which isn't annealed steel necessarily), then the low temp version will work.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

July 28, 2008

---

July 29, 2008

Sirs:

I still agree with Geoff, galvanneal refers to the zinc/iron alloy not the steel. This allows the coating to be formed without flaking off and the coating is more paintable. We are NOT talking about a high temperature.

Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

Hi. Unfortunately this can't really be clarified until Vinay returns to clarify it, because this is not a technical debate, it's a debate over what he meant :-)

But for now I'm going to even the sides by agreeing with Bill, because when someone wants to anneal something that is "full hard", I interpret that they're talking about trying to make hard steel milder; and if Vinay wanted to convert the galvanized sheets to galvanneal, I think he would have used that term or mentioned painting. But I'll try to raise Vinay to come back and clarify :-)

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
July 29, 2008