PDA

View Full Version : What is heat effected zone?



alan waters
06-02-2006, 03:21 PM
What is heat effected zone and what are the detrimental effects of it?

halbritt
06-02-2006, 04:01 PM
It's actually known as "heat affected zone" and abbreviated as HAZ. This sort of information is commonly available on the Internet. Ask google and you will be inundated with information on the topic. Here's something to start with:

http://en.wikipedia.org/wiki/Heat-affected_zone

76GMC1500
06-02-2006, 04:43 PM
Most metals undergo some sort of property change when heated above a specific temperature. For steels, you have the eutectic temperature. Above this temperature, the microstructure of the steel becomes austenitic (the steel becomes non-magnetic at its austenitic temperature). As it cools back out of the austenitic state, depending on the rate of cooling and composition of the steel, it will form a mixture of other microstructures including pearlite, bainite, martensite, cementite, and others. While welding, the metal heats and cools quickly which tends to form cementite. Cementite is very brittle, so areas heated above the eutectic temperature during welding of mild steel are referred to as the heat affected zone and have a different microstructure than the metal before the weld. HAZ's on mild steel are generally harder and more brittle than the parent metal if not heat treated after welding.

300 series stainless steels never leave their austenitic state, even at room temperature so their microstructure doesn't change during welding. However, they are affected by the heat in that within a specific temeprature range they begin a process called carbide precipitation where free carbon in the steel combines with chromium to form chromium carbide. This reduces the corrosion resistance of the stainless steel. So, unlike mild steel, the heat affected zone of stainless steel isn't a brittle or hardened area. Instead, it is an area of reduced corrosion resistance

For aluminum, it just gets weaker when heated above a certain temperature so the heat affected zone referes to the area weakened by welding.

alan waters
06-04-2006, 12:07 AM
I would beware of copyright infringments.

76GMC1500
06-04-2006, 04:07 AM
Did I infringe something? Those are all of my own words and I used no references.

gnm109
06-04-2006, 11:09 AM
I would beware of copyright infringments.


.................................................. .............................?

zapster
06-04-2006, 11:40 AM
heat effected zone...



anyone that comes within 5 feet of me with the rosebud propane torch in my hand....:angry:


:D

...zap!

76GMC1500
06-04-2006, 02:17 PM
Maybe the Wikipedia article was plagurized?

Sandy
06-04-2006, 02:51 PM
Maybe the Wikipedia article was plagurized?

Nah, there were credits there.

Don't worry about off the wall statements that are just thrown out there. ;)

pulser
06-05-2006, 12:37 PM
Attached is photo of an etched weld cross section showing the weld, HAZ, and unaffected base metal. You can see that the HAZ consists of lager grains than the unaffected base metal. Grain coarsening is a major issue in the HAZ of steels and many metals. Once heated in the austenitic temperature range, the grain size tends to increase with time. In general, a larger size grain structure has less toughness than a fine grain structure because there is a more direct fracture path along the few, straight, large grain boundaries. The grain boundaries are inherently weaker than the grain itself and they tend to contain impurities such as sulfur and phosphorus. The weld in this photo has a hydrogen induced underbead crack. This type of cold cracking is a function of the hard brittle coarse grain structure of the HAZ, the presence of dissolved hydrogen in the weld and HAZ, and a high tensile stress across the weld associated with a highly restrained joint.

It is martensite, not cementite, formed during rapid cooling of the HAZ is the major problem regarding brittleness of the HAZ.

Regarding 300 series stainless steel, 76GMC is correct, this steel remains austenitic and therefore does not form martensite and harden as do other steel alloys. Because it is not "heat treatable", a 300 SS weld may be quenched immediately after welding with no detrimental hardening.

76GMC1500
06-05-2006, 03:00 PM
I disagree about martensite. Martensite is a desirable microstructure formation in some applications because of its toughness. Martensite forms more readily in higher carbon steel, eutectoid and hypereutectoid steels. Martensite is also the last microstructure to form during cooling and forms at a very low temperature. Once you cool below, say 400 degrees C, the remaining austenite transforms into martensite. The rest of it has already formed pearlite, cementite, and bainite. Cooling fast makes more cementite so there is less austenite to form martensite with at the end of the process.

pulser
06-05-2006, 07:20 PM
76,
please check your sources and let me know if the following is incorrect.

As you say, martensite is a desirable microstructure. It is desireable for many applications because of its high strength and hardness. All hardened knives and tools contain martensite, but the additional process of tempering is generally needed to reduce the extreme brittleness and give toughness to the martensite.
Tempering is the process of reheating the quenched and hardened steel to a temperature below the austenitizing temperature, in the range of 300 to 600 deg. C, to allow some carbon to diffuse out of the martensite, resulting in improved ductility and toughness.
100% untempered martensite results in the highest possible hardness for steel, approximately Rc 68. Untempered martensite in the HAZ of weld is definitely undesireable, particularly if the weld has picked up hydrogen contamination, hydrogen embrittlement and delayed cold cracking may result. As a general rule, welding begins to require the precautions of preheat and postheat to slow the cooling rate and avoid excess martensite formation as the carbon content increases above 0.40%, or a 1040 steel.

Cementite is iron carbide. The eutectoid composition is the composition of pure iron carbide and is 0.83% carbon. Hypereutectiod steels are greater than 0.83% carbon. Slow cooling of carbon steel results in a lamelar structure composed of alternating layers of cementite and ferrite, called pearlite.

Please check out this link which discusses the continuous cooling transformation (CCT) diagram.
http://www.matter.org.uk/steelmatter/metallurgy/7_1_2.html

If you'll take a look at the CCT example/simulation for a 0.4% carbon steel, you will see that a rapid cooling rate misses the ferrite, pearlite, and bainite curves, and results in 100% martensite.

As you mentioned, higher carbon content increase hardenability. Carbon and other alloying elements increase the ease of hardening by pushing the nose of the CCT digram further to the right, allowing martensite to form with slower cooling rates. Hence air hardening tool steel, versus oil hardening, versus steel requiring a water quench.

In some alloys, quenching may result in martensite with some bit of retained austenite. A futher cryogenic process may force this retained austenite to transform to martensite.

76GMC1500
06-06-2006, 02:16 AM
It's going to be a while before I can get back to you, I'm going to have to get out my text books. You've got me interested.

76GMC1500
06-06-2006, 02:46 PM
It appears I interchaged cementite with martensite in my mind. Cementite is what you get when you go above 6.7 wt% carbon in an iron carbon alloy. Basically, the iron is saturated with carbon and the carbon has nothing to do but form its own grain between the iron grains.