February 2024 Volume 6

EQUIPMENT & TECHNOLOGY

The DFIQ unit approximate dimensions are 140” x 40” x 90”. The DFIQ system uses a quenchant developed to minimize duration of the non-controllable and non-uniform film boiling process when quenching. A DFIQ unit can be sized to meet the production requirements of a forge. Fig. 2 shows a hot forging placed on the loading table prior to quenching.

intensity of the heat extraction rate from the steel during quenching. It depends on the quenchant physical and thermal properties, quenchant agitation rate, part temperature, etc. The HTC was determined experimentally by using a special probe equipped with an array of thermocouples as developed by DANTE Solutions. The cooling curves obtained were used to calculate heat transfer coefficients when quenching forgings in the portable DFIQ unit. This data is critical to the success of DFIQ. 4. Results Twelve different production forgings were subjected to the DFIQ process during the project [3] . Some of these forgings are presented in Fig. 3. The components were forged with different plain carbon, alloy, and high alloy steels, ranging from 4 to 95 pounds, and representing different complex geometries. Material characterization of the three different alloy steels revealed all material mechanical properties improved. For example, mechanical properties of the DFIQ 4140 steel were the same or better than conventionally heat-treated higher alloy and while being less expensive than the baseline 4340 steel. In general, for forgings having ASTM grain size of 7.0 and greater after DFIQ, the DFIQ generally imparts either similar or better mechanical properties as conventional post-forging heat treatment. The effect of DFIQ on material mechanical properties is generally mixed for forgings having the grain sizes ranging from 2.5 to 6.0: some mechanical properties were improved or did not change. These findings can be attributed to the importance of the starting microstructure of the DFIQ process. During conventional hot forging process, the coarse grain structure of the billet is reduced and replaced by finer grains. Further refinement of the material microstructure takes place during the post-forging heat treatment process. The microstructure becomes finer every time the steel is heated up above the austenite transformation temperature and cooled down to a temperature below the transformation range of austenite to other microstructures (pearlite, bainite, etc.). This further refinement is usually absent when using DFIQ. Therefore, the initial grain size of the billet and the material grain size after forging operations are especially important when applying the DFIQ process. Since the grain size of forgings greatly depends on the forging temperature, controlling of the reheating furnace temperature becomes very important for avoiding the development of coarse austenitic microstructures. Therefore, induction heating that provides better control of the billet temperature is an ideal match with Intensive Quenching. It is important to note that part microstructure, and therefore material mechanical properties, after DFIQ, will be further improved in cases when the part requires additional heat treatment (for example carburizing, carbo-nitriding, etc.).

Fig. 2: Hot forging prior to quenching

2. Parameters used for evaluating DFIQ After the DFIQ process, the forgings were tempered to a specified hardness and checked for quench cracks by magnetic particle inspection. Forgings were then sectioned for metallurgical evaluation. Material microstructure and grain size were examined. Tensile and Charpy specimens were produced for determining the following forgings mechanical properties: tensile strength, yield strength, elongation, reduction in area and impact strength. Material microstructure and mechanical property data of the DFIQ forgings were compared to conventionally forged and heat treated parts. The DFIQ process proved successful for a variety of steel forgings of varying geometries and different grades of steel. It was demonstrated that the DFIQ can successfully produce microstructures, hardness and strength properties that meet or exceed product requirements. 3. Determining of Heat Transfer Coefficient During DFIQ When applying DFIQ, the quench should be interrupted when the surface compressive stresses are at their maximum value to avoid cracking. The optimal dwell time is determined using IQT’s proprietary software. One of the most important input parameters for the computer program is the value of the Heat Transfer Coefficient (HTC) in the DFIQ unit. The HTC characterizes an

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