Conditions for effective removal of binders (delubrication) vary widely from part to part depending on factors such as the type (and quantity) of binder used, part size and density, heating rate [1,2], atmosphere composition [1,3,5], atmosphere flow direction and displacement [4], additions for accelerated burn off, time at temperature and set temperature.
The delubricating section of a modern sintering furnace is equipped with several features to accommodate as many processing variables possible, given the constraint (in mesh belt furnaces) of a uniform transport speed through the delubricating, sintering and cooling sections.
In selecting a sintering furnace, it is first necessary to decide whether the furnace should have a rapid or conventional delubricating section. A rapid delubricating section has a short length where parts reside from 3 to 8 minutes, and is fitted with a premix burner having reliable control over the air/gas ratio through the range of high and low firing [6]. Rapid heating in the presence of a nonoxidizing atmosphere causes rapid binder vaporization without decomposition. When endogas atmosphere is used, controlled injection of air eliminates the need to fire the burner [3]. The rapid delubricating section is designed to remove vapor without binder decomposition and soot deposition. This design has the advantage of high productivity and low processing costs, and, therefore, a large number of such furnaces are installed worldwide.
On the other hand, this is not the best solution for several types of parts, especially highly alloyed slender high-performance parts, which require very close control during delubrication. An indirectly heated delubricating section is the preferred option in these cases, usually with two or more zones of control and multiple atmosphere gas injection points. A muffle is almost always used. Indirectly heated delubricating sections are designed for a longer residence time (typically 20 to 30 minutes) and allow better control over the heating rate and the atmosphere under which delubrication occurs.
Expanding operating options
Frequently, a furnace has to process parts for which it was not chosen. The addition of certain features to either type of delubricating section expands the range of successful applications.
For example, an extended narrow insulated tunnel leading to a rapid delubricating section allows a shallower heating curve. Apart from conserving energy, initiation of evaporation occurs earlier and the time for effective delubrication increases [1,2]. When processing bulky parts, the use of an entry end auxiliary burner bumps up the heating curve and helps to maintain the delubricating section temperature (Fig. 1). Under-belt firing, which causes the combustion products to flow through the belt (and the parts on it), displaces the layer of stagnant gas around the parts and reduces the tendency to soot. The use of a strategically placed internal door helps divert off gas to the chimney incinerator under different processing conditions. Provision of spare ports other than those provided for temperature, pressure and gas composition measurements prove very useful for gas injection or extraction if required at a future date, or for processing a particular part.
Careful consideration is paid to the space between the sinter section (which is muffled) and the open direct-fired rapid delubricating section. In this section, radiant heat from the sinter section heats parts coming from the delubricating section typically to a temperature of 850 C (1560 F) in a surrounding environment consisting of furnace atmosphere as well as combustion products. At higher temperature, residual binders emerge and decompose. The danger of surface oxidation leading to a matte finish also exists [5]. An exhaust chimney in this area with a damper optionally interlocked to a surge in furnace atmosphere flow provides the operator with another control with which to fine tune processing conditions for particular parts.
When a conventional indirectly heated delubricating section has to be used to process parts that would benefit from rapid delubricating, air, moisture (usually through an humidifier) or exogas is injected through one or more of the multiple injection points on the muffle, depending on the size, density, belt loading and binder content of the parts being processed.
The sinter section is provided with a muffle in furnaces equipped with either a conventional or a rapid delubricating section. The muffle is heated either by electric heaters running across the width of the muffle above and below, or, in the case of gas-fired furnaces, by self-recuperative burners preferably firing within radiant tubes.
Electric heating is the preferred option for close temperature control because the placement of elements is more uniform across the length of the muffle and also it makes it possible to install several zones of control across the length and above and below the muffle. The sinter section of a typical 450 mm (~18 in.) belt width furnace designed for processing critical high-alloy parts would be equipped with eight control zones, four above the muffle and four below (see lead photo).
Ceramic muffles are increasingly becoming the preferred choice over metal muffles because they do not distort, hump or scale over the heaters, or have a short service life of only 2 to 3 years. The normal effective service life of ceramic muffles is more than 8 years. The overall cost of ownership is lower, and other attendant benefits include lower downtime, less belt damage, the ability to operate at higher temperatures and not having to open up the furnace often. The use of ceramic muffles does not preclude the incorporation of atmosphere injection and gas-sampling points, which is done through specially designed shoes.
Ceramic muffles are not as gastight as metal muffles because they consist of segments cemented together, which can develop fissures due to frequent heating and cooling. For this reason, furnaces that use ceramic muffles are constructed with a gas tight casing, and a low flow of nitrogen (typically 1-3 m3/hr depending on the furnace size) is purged into the heating annulus outside the muffle. The expense of this low flow of nitrogen is insignificant compared with the replacement cost of metal muffles. Also, a spin-off benefit lies in the blanketing of the metal heating elements. NiCr coil elements have lasted up to 8 years in a furnace operating between 1120 and 1150 C (2050 and 2100 F). Normally, high-temperature oxidation would cause earlier failure. This also makes the need to install individual heater failure indication redundant.
Heating characteristics
The distribution of heating power is designed to bring the parts up to sintering temperature as fast as possible. The elimination of a preheat zone prior to the sintering zones (ostensibly for less distortion) does not seem to make a difference, possibly because preheat is achieved between the delubrication and sintering sections.
A quicker heat-up capability allows the belt to run faster for a specified time at sintering temperature, thus increasing productivity. The power at the end of the sinter section is increased marginally to compensate for the dip in temperature due to radiant heat loss through rear opening (Fig. 2).
The sintering section is followed either by a heated carbon restoration zone maintained typically at 950 C (1740 F) for improved control over carbon restoration or an unheated insulated section to reduce the rate of part cooling.
The bulk of the furnace atmosphere is injected into a water-cooled primary cooling chamber placed at the rear end of the furnace. An adjustable gas inlet manifold controls the quantity of gas that flows into the furnace and toward the cooling section. Heat is exchanged between the hot parts that emerge and the cold gas. The gas injection design allows a degree of control over the cooling rate of parts.
Following the primary jacketed cooler are additional cooling jackets built in sections for easy transportation and replacement. The walls of the inner muffle are protected against scaling by a diffusion coating that does not inhibit heat transfer. Water inlet is from below, and overflow is from the sides to allow the use of removable top covers that facilitate inspection, cleaning and pressure testing. Water flow is made turbulent to improve heat extraction.
Accelerated cooling (not amounting to sinter hardening) previously achieved using a fan within the cooling section, or by extraction of gas by a blower through a heat exchanger prior to blasting over the parts, has been replaced by an equally effective impingement cooler that uses the kinetic energy of the incoming gas. This design eliminates the possibility of air ingress, as well as the energy consumed by fans.
More than one of these coolers are dispersed along the length of the cooling section with gas distribution control to optimize results. Accelerated cooling is believed to be the reason for improved part properties of parts processed in a furnace with this feature compared with those processed in an earlier conventional furnace (Fig. 3). The tensile properties of parts subjected to accelerated cooling shown in the table generally are 10% higher than those of conventionally cooled parts.
An attendant benefit of accelerated cooling is that parts do not oxidize to the extent they would in furnaces not similarly equipped when air enters the cooling jacket for whatever reason (shop draft, insufficient flow, etc).
Sinter-hardening modules (also known as variable rate rapid cooling, or VRRC, modules), when required, replace the primary cooler and are preferably used with a prior carbon restoration type zone for a degree of control over the temperature of different types of parts and loading density. The VRRC module is designed with the help of computational fluid dynamics (CFD) software (Fig. 4). Cooling rates of 4-5 C/s (7-9 F/s) are routinely achieved (on single layer loading of parts, 45 kg/m2, or 9 lb/ft2, of the belt) using the conventional VRRC design where the furnace atmosphere gas is recirculated, cooled and impinged on the belt. Advanced designs, appropriate for walking beam (Fig. 5) and roller hearth furnaces, yield significantly higher cooling rates, which are being characterized to obtain a higher percentage of martensite and bainite conversion for parts having lower alloy content.
Process control
Control over the amount and direction of atmosphere gas flow through the sintering furnace is an important consideration for sintered part quality and consistency. This is achieved by adjustable gas injection manifolds coupled with a versatile gas flow panel having multiple gas streams and switching devices, as well as eductors in the front and/or rear chimneys.
Traditional control over flow direction has been via sensing temperature at either end or one end of the furnace looped to chimney damper control. Experience with pressure-differential control has shown mixed results, especially when the gas flow is lowered to reduce processing cost. A prototype velocity control system is presently under investigation.
Modern sintering furnace controls are usually run through SCADA software on a PC in communication with a PLC for control of temperature, gas flow rate and direction, atmosphere composition, cooling water temperature, belt speed and for interlocks. Process visualization, acquisition and recording of deviation conditions and reporting in several Windows type formats are usual features of this system. Riding piggyback on standard SCADA software is in-house generated TPM software, which can be used as a tool for furnace maintenance. Activities to be carried on the furnace at regular intervals are input, and the software reminds the user using a pop-up screen. The software generates all completed, rescheduled and uncompleted tasks as an output and updates the master scheduler sheet.
The software has a master history card, where all breakdown details and modifications carried out on the furnace can be entered. The software automatically calculates the "furnace stop" hours for the day and calculates OEE and OPE for various losses and stoppages of the furnace. From the history card, various reports can be generated, such as a spares-consumption report and cost of spare parts consumed including labor costs, in text and graph formats.
Predominant operating costs are energy, atmosphere, amortization and maintenance. The use of insulation that causes a skin temperature of 65 to 70 C (150 to 160 F) at an operating temperature of 1120 C and a shop ambient of 34°C in N2+H2 furnace with a gas-fired rapid delubricating section followed by an electrically heated sinter section consumes at a total of 355 kWh/ton electricity and 1kg/hr of LPG (propane+butane, or 11,500 kcals/kg).
Reducing operating costs by reducing gas consumption should be done with caution because the quality of parts could suffer due to sooting, oxidation and inadequate binder removal. Curtains in the front and rear help reduce gas consumption as do inclined entry and exit vestibules. A rule of thumb used by a bulk manufacturer is to reduce the gas flow until the belt comes out gray, but the parts are bright. Restricting the opening height and using a flat top muffle (if ceramic) helps reduce consumption.
Operating a continuous furnace continuously even with fluctuating loads extends furnace life significantly and reduces maintenance costs. IH
