CARBURIZATION

Carburizing is one of the most commonly performed steel heat treatments. For perhaps three thousand years it was performed by packing the low carbon wrought iron parts in charcoal, then raising the temperature of the pack to red heat for several hours. The entire pack, charcoal and all, was then dumped into water to quench it. The surface became very hard, while the interior or “core” of the part retained the toughness of low carbon steel.

Carburization Testing

Laboratory carburization testing must be carried out in some approximation of the industrial atmosphere of interest. The test temperature should be similar to that anticipated in service. In addition it would be a good idea to include thermal cycles about like the expected service conditions⁶. Finally, duration of the test is important.

Carburization resistance depends upon the chromia scale, the silica subscale, and the alumina scale in some alloys. For this reason the test atmosphere should, in our opinion, contain an oxygen partial pressure comparable to the expected service atmosphere, in order to form a similar protective scale⁷. One may also wish to consider nitrogen, as nitrogen the atmosphere reacts with alloying elements such as chromium, and may affect carburization.

There have been laboratory carburization tests run in an atmosphere of hydrogen — 2% methane, with no control of oxygen partial pressure. In this environment the alloy will not develop much of a protective scale. Such an atmosphere is one way to achieve the objective of actually carburizing most alloys.

Very small amounts of oxygen can form enough alumina or titania scale, for example, to inhibit braze flow in many vacuum furnaces. Alloy 800H contains enough titanium to turn light gray in some vacuum heat treat furnaces.

In order to braze even stainless steel (with no Al or Ti) in hydrogen it is normally considered that the dew point should be – 60°F (– 51°C) or lower⁸. This is necessary to dissociate the oxides of most alloying elements. Alumina and titania will not be dissociated by this atmosphere. One might expect that grades such as N06601, N06025 and N0811 would form aluminum and titanium oxide films in a nominal hydrogen — methane atmosphere. Such films may affect carburization.

Recent work by George Lai, in H₂-CH₄, ranks several alloys in the same order as does service experience. Oxygen partial pressure was not indicated. Dr. Lai’s ranking of wrought alloys, from best to worst is: Haynes ^® 214, RA 602 CA ^© , Incoloy ^® 803, 800H, and 310 stainless. The ranking is the same, whether based on weight change or on measured depth of carburization. Alloy ranking is approximately in accordance with their aluminum + titanium contents. Al and Ti are is the elements most likely to form a scale in this test. We might infer that these methane – hydrogen test results could have some relevance to performance of alloy fixturing in a vacuum carburizing furnace. In vacuum carburizing very small amounts of oxygen are present from the furnace leak rate, if nothing else.

Carbon Content, weight %, Before and After Testing

Alloy	Original	Final	Increase	%Al
214	0.042	0.50	0.008	4.5
RA 602 CA	0.19	0.36	0.17	2.2
803	0.084	0.99	0.91	0.3
800H	0.082	1.23—0.95	0.86—1.14^a	0.4
310	0.08	2.73	2.65	0.05

^a range of five specimens

When the atmosphere does simulate that of industrial interest, carburization testing may require long exposure. There is some period of time during which significant carbon absorption does not take place. Experience related to us from one furnace company indicated that the test had to be run for at least 1000 hours, before their results correlated with service experience. Their test data, shared with us, is below.

The tests were conducted in an electrically heated industrial carburizing furnace. The higher temperature results, 1900°F (1038°C) are from a composite electric heating element made of the five alloys shown, and the 1750°F (954°C) results are from plate samples exposed to the actual furnace operating temperature. In both cases the total exposure hours were distributed as follows: 20% of the time in endothermic gas enriched with natural gas to carbon potential 1.0 – 1.2%C relative to iron, 70% of the hours in nitrogen, and 10% of the time reflected air burnout cycles at 100°F (56°C) reduced temperature. Various depth of cuts were machined in the samples and the carbon contents analyzed. Results here are reported at 0.045” (1.14mm) depth on the element and 0.20” (0.508mm) depth on the plate sample.

	1900°F (1038°C) 2260 hour exposure	1750°F (954°C) 4300 hour exposure
alloy	%carbon	%carbon
RA333	1.53	0.344
RA330	3.03	0.443
617	2.86	1.6
601	2.98	1.096
600	1.56	- -
310	- -	3.92

Vacuum Carburizing

Vacuum carburizing presents a different environment. Behavior of alloys in conventional atmosphere carburizing do not necessarily predict how they will perform in low pressure carburizing. Still, there is always a small amount of oxygen in a vacuum carburizing furnace. Some may be introduced through traces of acetone in the acetylene used as a carburizing gas. The leak rate in the furnace will always permit some oxygen to be present. While not enough to form a stable chromia or silica scale, the 2.2% aluminum alloy RA 602 CA will form an alumina film on the surface. It is this oxide that is responsible for the alloy’s resistance to carburization in this environment.

RA 602 CA alloy baskets designed for low pressure carburizing, This captive shop processes transmission sprockets and gears at 1650°F (900°C) in Abar Ipsen furnaces with integral oil quench. The carburizing gas is acetylene. Baskets are stacked three high, and will see two cycles per day on average.