Heat Treatment

2 Building a Small Heat Treatment Oven

When making a small tool that only requires that the tip of the tool is hardened, then it is easy to just hold it in front of a blowtorch until the correct temperature is reached and then quenched. Something a bit bigger, or more importantly of a much thicker cross section, will require soaking at a high temperature for a longer time to impart the hardness to a greater depth. For example, O1 tool steel requires soaking at 780-820°C for thirty minutes per 25mm thickness, with preheating at 3-500°C before elevating to the higher temperature. To get the best out of the steel the nearer we get to these ideal conditions, the better will be the result. Of course, you could wave the blowtorch over your home-made tool until it reaches red heat and then hopefully keep it hot enough by constantly moving said torch, then immediately quench it, but at best it will most probably be just the surface being hardened. It takes time for the heat to penetrate from the surface and alter the structure.

The material data sheets. They will give the correct temperature for soaking and subsequent hardening of each particular type of steel being used.

It is possible with small items to produce enough heat from a blowlamp to effectively harden the steel.

Although your blowtorch, even the smallest, is capable of attaining a flame temperature well above most hardening temperatures, out in the open any heat put into a piece of tool steel, or anything else for that matter, will be lost as soon as you move the torch to another spot. Steel, being a reasonably good conductor of heat, will tend to wick away the heat as it is applied from the torch. At best you could keep a large item hot by continuously moving the torch around to cover a bigger area, but this is wasteful of gas. You are not only supplying the heat to raise the temperature, you are having to supply the heat to replace what is being lost through conduction and radiation. By enclosing the gas flame inside an insulated area it will retain more of the heat and give a more even spread of the heat over whatever you are heating, rather than directly from the flame tip. For an even hardness it is imperative that the temperature is even throughout the item that you are hardening. Of course, reading the above it will be obvious that if you only have a small gas blowlamp, with the aid of a heat-proof and reflective container or chamber it would be possible to tackle larger items that would otherwise be impossible to harden and temper with the small torch on its own.

BRAZING HEARTH

If you are already familiar with and carry out brazing to any degree you may well have a brazing hearth at your disposal. You will also be aware that when carrying out brazing operations, if you can nestle parts being brazed in pieces of broken firebrick you will contain the heat around your brazing items and the job will be done in a shorter time. You may also get away with using a smaller gas torch than you otherwise would have needed. The brazing hearth will not be ideal if you have a lot of hardening and tempering to do, however I often have a need to harden and temper thrust washers so I drill and turn the end of a piece of suitably sized silver steel bar to the correct inside diameter. I can then part off the washers to the desired thickness and quantity. All that then remains is to heat them on the brazing hearth to carry out the hardening and tempering process. For small items such as these washers, the brazing hearth works quite well when carrying out the tempering process. The hearth has a rail on which the torch can be placed, with the flame going over the hearth, by holding the washers in the flame of the torch, usually suspended on a piece of thin rod or wire with the end bent into a hook to keep the hands clear of the heat from the torch.

A purpose-made brazing hearth is a good place to start when hardening and tempering.

Small items can be nested within pieces of firebrick to help retain the heat from the blowlamp and even out the temperature.

As the tempering temperature is much lower than the hardening temperature, in this case under a third as I tend to temper these washers at a blue colour that equates to around 270°C, it does not take so much heat to raise the washers up to reach the correct colour. Holding the rod in hand, as soon as the washer is blue I can immediately dunk into the bucket of quench water. The other advantage of holding the item in the hand is the fact that if the temperature is going up a bit too fast it is easy to move the item away from the flame to slow the rate of temperature rise. It is very easy to overshoot and temper at a higher temperature/colour than you wanted. After removing from the quench, the oxide colour will tell you whether you were successful or not as immediately the item is quenched the oxide colour will stop changing.

SIZE MATTERS

If you are only working with small items to harden and temper, then there is no point in building a gargantuan furnace/kiln/oven; it will not only take a long time to warm to working temperature and it will use a lot more gas to maintain the heat levels required for the tasks in hand. The basis of making an oven is very simple. For strength, a steel outer case is a practical solution, although I have seen ovens built with a framework of Dexion angle to hold firebricks in the required place. To prevent too much heat escaping before it has had a chance to heat your metal, some form of insulating material will be required. Also you will require some form of port built into your forge/ oven so that you can either temporarily attach the nozzle of your gas blowtorch, or hold a purpose-built gas burner, of which there is a plethora of designs on the internet that would be easy to build with a modicum of practical skills.

When building an oven, a means to hold the gas burner will be required. This was made from steel tube with three tapped holes for the retaining screws.

This was welded into position on the outer shell of the oven. The screws afford a limited degree of adjustment as to where the flame will go inside the oven.

CONSTRUCTION MATERIALS

Under no circumstances should you use household bricks or any concrete products to build an oven as any moisture trapped in them will turn to steam when heated and will most probably send lumps of material across the workshop as the trapped steam tries to escape with explosive force. It would easily take an eye out. I have heard of people using offcuts of thermalite blocks scrounged from builder's skips and, although they might be suitable for use as a firebrick, I have my doubts. There is no telling what other stuff has been put in the skip with them and they could be contaminated with all sorts of chemicals and compounds. This contamination may not appear on the blocks, but on heating they may well vaporize from the heat from the blowtorch and cause undesirable effects on the items you are hardening. More importantly, they could be very detrimental to your health and any bystanders in your workshop.

Cutting the insulation for the end of the oven from a sheet of soluble fibre insulation. Although the fibres are supposed to be safe, always wear a suitable breathing mask when handling this type of material.

Fitting the insulation inside the oven, marking to length before cutting as a snug fit is required to keep the heat inside the insulation.

Firebricks are ideal, although they have a tendency to absorb quite a bit of the heat being used. Those made from vermiculite tend to reflect more heat and have the advantage that they are not so heavy, and as always there is a downside in that they are not so robust as those made from clay and cannot stand such high a temperature. Again the danger from heating moisture trapped in bricks must be kept in mind. Under no circumstances use firebricks that have been left out in the wet. Apart from the risk of possible flying lumps, the steam will cool the heating process.

Kaolin or ceramic wool blanket is an excellent heat-reflecting material and is ideal to use for this project. The disadvantage of kaolin is that it is very prone to mechanical damage as it is made from many fine fibres, hence the name. It is generally available in two thicknesses, 25mm and 50mm, and two densities, 96 kg/m3 and 128kg/m3. The higher density blanket has a higher thermal insulation value, but for the purposes of our oven, the lower density would suffice. There is a health risk associated with kaolin wool and ceramic fibre as it has been deemed to be cancerous through its fibres, but as it was tested by overloading rats with the fibres until their lungs were full of the stuff, it is not surprising that they got cancer. Having said that, providing that sensible precautions are taken when handling the loose product, i.e. wear gloves and more importantly a breathing mask and work in an airy place, then the risk should be eliminated or lowered to a much safer level. The main risk is when cutting the product to shape; once installed and the kiln or oven is being used properly the risk is minimal. It is an advantage to the longevity of the product to coat it with a painted or sprayed on coat of what is known as rigidizer. It is carefully applied with a brush or sprayed on to the surface of the fibres and allowed to air dry. After several layers have been added in this way and dried. the oven can be fired up briefly and allowed to cool down to make sure all the moisture is removed before a full firing is undertaken. There is a slightly more expensive version of the standard ceramic...