Surface Burning Can Result From A Number Of Factors. We Can Solve.
This week continues our discussion of heat treating problems and how to solve them. What we call “surface burning” actually describes the melting of a part’s surface. Obviously this is something you want to avoid. Surface burning happens when too much heat penetrates the area for too long – which can result from a number of factors. We see this problem with both induction and flame hardening, but may be more common with flame. Keep reading to understand why.
The first place we look to correct this problem in flame hardening is in the flame ports of the flame heads you are using. Flame ports take the bulk of the heat as the oxygen and fuel gas mixture flows through them. Ideally they are all operating identically, all producing the same flame intensity and shape. But over time some may degrade and burn hotter than they should. The problem happens most often in progressive flame hardening designs where the flame head is slowly moving over the hardening area, rather than in spinning machines where the part moves and usually at a pretty fast clip.
If a flame port in a progressive head burns hotter as it moves, one area is going to get too much heat sinking too deep. It’s all about heat transfer from the flame coming out of the flame port to the steel. To get an eighth of an inch hardness you may need to heat to 1900 Fahrenheit on the surface, when melt for that material is 2000 degrees. Hotter flames out of degraded ports can easily send you over melt point.
If flame ports check out okay, look next to the fuel mixture. Is the flame too oxygen-rich? The more oxygen flowing through the flame port, the hotter the flame coming out of it. The more gas, the cooler the flame. Make sure you have the proper ratio of oxygen to fuel so the temperature of the flame reaches the appropriate level for your material and mass, and that flows – that is, the volume, or velocity of the oxygen/fuel mixture – also give you that temperature consistently. Flame heads also need to be manufactured out of particular types of brass and copper that can withstand long exposure times to high heat. Because these materials conduct heat, these flame heads stay cooler and are less susceptible to degradation.
All flame heads need to have cooling water flowing through them to handle the quantity of fuel and oxygen in the flows. When you have many flame ports producing flames that are all running 1500 to 1900 Fahrenheit, that’s much more heat than just using a torch from the local welding supply. These are specially designed units that not only handle the heat demands necessary to achieve desired hardness but do so over and over for years, with minimal maintenance.
Improper flame head design can also lead to surface burning. Often we find that parts to be hardened have areas with more mass than others, but they both need to be heated evenly at the same time. For this type of application we add more flame ports to the head in a design that allows that greater heat to go only to the denser mass and correspondingly reduce the number of ports covering the less dense areas.
It’s easy to conclude, then, that surface burning issues almost entirely reside in some sort of flame head problem – either the ports themselves, the type of flame issuing from them, or the design and position of the head. Some of the most important design work we do begins with the customers’ drawings of the part they have and coming up with the flame head that is going to heat it most consistently. Each design we do has to be specifically tailored to the requirements of the customer’s unique part.
Next week we’ll begin getting into the nitty-gritty of fine tuning flame hardening techniques – how to tinker with the system until your hardness levels aren’t too soft, and aren’t too hard, but just right. As always, you can reply or reach me at firstname.lastname@example.org or call Flame Treating Systems (919)956-5208 if you have a particular flame hardening question or problem you’d like to discuss.
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