Quenching of Tool Steel

After austenizing your tool steel component, your tool will need to be quenched. Quenching is the process of rapidly cooling the tool steel below its critical temperature. This rapid cooling of the tool traps the alloying elements within the tough iron matrix. If the cooling rate is not rapid enough, then the tool will not attain the proper hardness. If the cooling rate is too fast, then the tool could crack due to stress.

There are many types of quenching methods. Quenching of tool steel usually involves submerging the tool into a quenching medium. Each grade of steel may have more than one type of quenching method. Common methods of quenching include water, oil, air, salt brine and vacuum. Each quenching method cools the steel at a different rate. Water is the fastest method and still-air is the slowest method

The method of quenching is chosen based on the speed that the tool steel must be cooled to below its critical temperature, The size of the tool to be quenched also plays a role in choosing the quench method. A thicker section will usually need to have a faster quench rate than a thin section. This is due to the ability of the tools mass to retain heat.

Care must be taken during the quenching process to avoid bending or warping. As a tool is lowered into the quenching medium, it should be inserted in a vertical fashion. This is especially true for long thin tools. If a tool is lowered into the medium at an angle, then one side will cool sooner than the other side and warping will occur.

The tool should also be lowered into the medium quickly to attain an even quench rate from the top to the bottom of the tool. Uneven quenching will cause differences in hardness and internal stresses, which could cause cracking.

Water quenching is the fastest method of quenching. This method is used only on a few low alloy tool steels and some carbon steel grades. Water quenching is too fast for most grades of tool steel and will cause cracking. The tool should be removed from the water a few minutes after the boiling of the water ceases.

Oil hardening is a more common method of quenching. It is used on many tool steel grades of low alloy content and some large sections of high alloy content grades. The tool must be lowered into the oil quickly to minimize the flash that will occur as the oil ignites. If a tool is lowered slowly into the oil then the heat from the tool will ignite the oil and the surrounding oxygen will allow the oil to flash or burn. By rapidly submerging the tool into the oil, the tool is deprived of oxygen and the flash is minimized. The tool should be submerged in the oil only for a few minutes after the smoking and rolling of the oil subsides. A large enough quantity of oil will need to be used to avoid overheating of the oil. When overheating of the oil occurs, the flash point of the oil is lowered. If the flash point is lowered then the tool is not able to cool quickly enough and the oil can burn for a long period of time.

Air quenching is the most commonly used quenching method on many tool steels. It is a relatively safe method of quenching and is used for quenching thin tools made from most tool steel grades. In most cases the air quenching is performed in still air with no circulation. There are a few instances when an accelerated air quenching process is beneficial. An accelerated air quench is the application of adding moving air to the standard air quenching method. This is generally done by the use of a fan. On complex shapes, care must be taken to the position of the fan. The moving air should be directed to the thickest section of the tool. This will help achieve a move even quenching rate through out the tool.

Salt brine quenching is performed by placing the tool is a solution of salt brine. The salt brine provides for a very even quench rate in the tool. Salt brine quenching is usually used only in commercial heat treating operations due to the cost of maintenance of the solution and equipment used.

Vacuum quenching is only performed within a vacuum heat treating furnace. This method does eliminate the need to move the tool in order to quench it. This process is usually only performed in commercial heat treating operations. The cost of a vacuum furnace is cost prohibitive for most toolmakers.

Regardless of the quenching method used, no tool should be allowed to cool to room temperature. Cracking could occur if the tool cools to room temperature. Ideally the tool should be allowed to cool to 125-150 degree F and then it should be tempered.

Continue to Tempering of Tool Steel to see how to obtain the proper hardness of your tool steel and to relieve the brittle condition of your quenched tool.