|Fig. 1. Example of random loading of fasteners on a mesh belt prior to carbonitriding|
We begin with an interesting tale of what happens if we naively follow along. Just ask the four young Oysters who become enthralled with the seemingly idle chatter of the Walrus and the Carpenter, ending up as the main course at dinner.
“The time has come,” the Walrus said, “To talk of many things: Of shoes – and ships – and sealing wax – Of cabbages – and kings – And why the sea is boiling hot – And whether pigs have wings.”
The Walrus and the Carpenter, Louis Carroll, Through the Looking-Glass and What Alice Found There, 1872.
Atmosphere gas carburizing is a process so familiar to most heat treaters it is too often taken for granted. We trust our oxygen-probe readings to keep us safe, and we expect the outcome of the process to never change. But occasionally we get in trouble, and when we do, valuable lessons emerge. Let’s learn more.
We will start by looking at various external and internal factors that can affect the carburizing process, uncover issues related to process and/or equipment variability, discover where the pitfalls might lie and talk about what we can do to avoid them.
Many times, variation in case depth and other carburizing problems can be traced back to how parts are loaded in baskets and fixtures. Loading arrangements generally fall into one of two broad categories: weight-limited or volume-limited. In either case, when loading parts in furnace baskets or onto racks, our first instinct is to maximize loading efficiency. However, as heat treaters must also be concerned with proper part spacing (i.e. positioning parts within the load for optimal heat transfer), atmosphere circulation, temperature uniformity and heat extraction during quenching (to minimize distortion). And while trial and error is often the most prudent path, we must also take into consideration:
- Furnace-induced factors (often a function of the style of furnace in use). Being aware of the process limitations induced by a given design is an invaluable aid when things go wrong.
- Part geometry and orientation factors. We need to ask ourselves questions such as, “How much space should be left between parts?” and “Is random loading (Fig. 1) or nesting possible or even prudent?”
For example, bearing races of various diameters – a typical volume-limited load configuration – are often nested inside one another, producing an “optically dense” workload that is difficult to uniformly heat in many cases. In this instance, the cycle must be adjusted to allow enough time for the interior parts to reach temperature. Here, the furnace fan (type, speed, rotational direction, location) plays a significant role in the heating process. Fasteners are another example of where random loading in either continuous or batch-type (Table 1) units is most often used to handle the sheer volume of parts to be run. In this case, atmosphere penetration throughout the load, cleanliness of the parts entering the furnace and allowing adequate time at temperature are considerations that must be factored into the process. If parts are not bulk loaded, a good rule of thumb is that the gap around a part should be no less than 25% and no greater than 75% of the parts’ envelope diameter (Table 2).
|Fig. 2. Internal furnace contamination – sodium deposits in the form of a glassy coating|
|Fig. 3. Low case hardness|
Carburizing Process Problems and Their Solutions
Inadequate Case Depth
Not achieving the desired case depth (Fig. 3) can be due to a number of factors, some of which are carburizing at too low a carbon potential (i.e. too lean a furnace atmosphere), partial or complete decarburization of the part surface from air infiltration due to a leaky furnace, processing at the wrong temperature perhaps due to malfunctioning or improperly located thermocouples, retained austenite in the case region or a “slack” quench.
Steps that can be taken to correct these maladies include increasing the carburizing potential of the furnace atmosphere (particularly if boost/diffuse carburizing is being performed), changing the carburizing process (e.g., carburizing and slow cooling followed by a subcritical anneal prior to reheat and quench), subzero treatments and selecting the proper tempering temperature.
Shallow Case or No Case Depth
Producing shallow case depth or areas where there is no case development points to incomplete surface preparation prior to carburizing, the presence of surface contaminants or possibly the misapplication of selective carburization methods (i.e. stop-off paints or poorly adhering copper plate). Another area of concern is how the parts are being received from upstream operations. Dirty dunnage and suspect transport methods may add a level of contamination (e.g., rust) that is unacceptable to the carburizing process.
Solutions to these problems include controlling the cleaning process, cleaning the parts washer as well as replacing its solution on a frequent basis, and handling parts with clean gloves.
In part 2, we will discuss problems associated with retained austenite, decarburizing/de-alloying, intergranular oxidation, case leakage, case cracking/separation, case crushing, untempered/tempered martensite effects and other issues. IH