Heat Treating: Precision Surface Hardening of Steel Rolls
December 1, 2006
Flame hardening technology has come a long way. Once a process controlled completely by sight and feel, precision surface hardening is now (as the name implies) a precision process controlled and monitored by the latest in technological advancements.
Long gone are the days when a machine operator sat in the dark corner of a shop staring at the cherry red glow of a part being flame hardened. Typically the “artist” would look through dark welding glasses and make the decision that the color “looked good” just before water spray quenching completed the job. The “artist” was often questioned how he knew the temperature of the part. The answer was, “you just know.” This was acceptable, as no one wanted the job! It was hot, dirty, wet and sometimes considered dangerous.
Progressive-Spin HardeningProgressive-spin hardening is the method employed for hardening rolls, tubes, cylinders and piston rods. The part is loaded into a lathe either vertically or horizontally. A custom-designed ring-burner encircles the part (Fig. 1). The coupling distance between the part and flame-head and number of heating tips is determined by the material, hardness and case-depth requirements. Following the heating ring is the quench. As the part rotates, the heating ring and quench is traversed across the face of the roll at a given speed. For years, this has been adequate, and for many applications, it still is.
Process ControlOver the years, the control of flame hardening continues to be questioned. Other forms of heat treating have gained all the attention as they are considered the bulk of heat-treating processes. The flame-hardening industry is quite small, and over the past few years, several companies have closed their doors. Improvements, however, have been made to the process. Regulators, flow controls and optical pyrometers have all made significant strides in helping to control this surface-hardening process. The developments in recent years have now taken the control to the next level.
Precision surface hardening now utilizes digital high-speed optical-resolution non-contact infrared thermometers as the key for control and repeatability. Each pyrometer unit is equipped with laser sighting, which allows pinpoint placement for extreme accuracy of surface temperature monitoring. The preheat temperature, austenizing temperature at quenching point and residual temperature are all key elements for controlling the hardness and case depth (Fig. 2). Each unit has its own digital readout and is tied into a virtual chart recorder for traceability. The color monitor provides easy viewing while the flash-media card allows for permanent record storage. The operator is able to scroll back through the recorder to view the heating cycle at any time. When the surface-hardening process is complete, the heating-cycle file is downloaded to a flash-media card, which is then transferred to a CD for permanent record storage.
Burner Design and MetallurgyA high-intensity burner system was designed to produce the optimum in “Dual Zone” case depths on steel rolls. Materials such as 1045, 4140, 4150 and 4340 are commonly used. For this application, however, 52100 was chosen (Fig. 3). This high-carbon-content material allows for superior surface wear resistance. The hard case is then supported by the deep hardness gradient below. In some applications, 52100 Modified is used. The additional case depth is sometimes required to support the extremely high pressures from the rolling operation of flattening and leveling steel coil. This type of heat treatment and case depth extends the life of the roll by offering multiple regrinds. It also supports surface coatings such as chrome plating.
Thermal ProcessingThe case depth and transition zone is important. The core of the material, however, must be suited for the actual application. Raw material choices from hot-rolled steel or forgings followed by a spheroidize anneal, normalize and temper or quench and temper are all possible options.
The heat-treating process for 52100 and 52100 Modified material is followed with a cryogenic treatment. Usually performed between to –120°F to –200°F, this process assures that complete transformation has taken place (Fig. 4). The deep-freeze process eliminates retained austenite, a soft unstable microscopic phase, and converts it to a denser, harder, more stable structure known as martensite. If left untreated, transformation of austenite can occur during finishing or in service. This transformation may lead to cracking, followed by actual spalling of the roll surface.
TemperingOnce the roll surface has been hardened and cryogenically treated, it is tempered. Typically this is done by placing the roll in a furnace at a specified temperature for a specified time usually between 300°F and 1200°F for four to eight hours.
Prior to tempering, the as-quenched structure is very hard and brittle and lacks good mechanical properties - namely toughness and ductility - and is highly stressed and dimensionally unstable. Proper furnace tempering, coordinating both furnace time and temperature with the chemical grade and specification requirements, optimizes surface properties while meeting the required surface hardness for a particular application.
SummaryWorking closely with the manufacturers on the latest technology has been the key to both the success of the precision surface hardening process and the end product to the customer.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: cryogenic, retained austenite, non-contact infrared, case depth