This article takes a look at the benefits, needs and objectives of a quench-press, and explains why manufacturers who deal with distortion during heat treatment may want to consider what a quench-press could do for their products’ quality and cost.
There are many different ways to use a quench press, depending on your objectives. Four of the popular objectives are listed here.
1. Preserve the accuracy of the part during the hardening operation
2. Recover accuracy lost during carburizing
3. Improve roundness and flatness resulting from the release of strain energy when the parts are removed from the turning chucks
4. Force a particular diameter of the part to a fixed dimension – typically a bore dimension
Table 1 shows two examples of part dimensions before and after quenching – one for a carburized bearing outer raceway that fell victim to a fixturing failure during carburizing and the other for a 52100 through-hardened part.
The Driving Needs
The last two decades have seen significant changes in the designs of many heat-treated parts. Whether you manufacture automotive parts, saw blades or bearings, there generally has been a common theme that could be described as "higher load capacity, longer life, less metal – all at a lower cost." Modern 3D modeling and finite-element-analysis software have enabled the product designer to remove metal from the part design that is not mission-critical and create the physical design that produces the needed strength, life, rotational inertia and weight. Frequently, these new parts present heretofore unseen challenges to the heat-treatment process, especially when the total cost to produce the part needs to be lower.
When these new parts involve case treatments, there typically is a need to assure that the parts will achieve dimensional conformance when the case treatment is not ground or only has a bare minimum of material removed. Often a quench press is required.
In a JIT manufacturing environment, the cost of scrap due to excessive warpage has put the focus on the heat-treating op-eration in part due to the long delays in obtaining new forgings or special raw material. The cost of scrap and the potential for losing customers has created a heightened interest in quench pressing and the benefits it has to offer.
A typical list of "Needs and Objectives" that a quench-pressing operation can help satisfy is noted below.
Reduced total part cost as a consequence of:
- Reduced scrap
- Reduced "air grinding" time necessary to prevent grinding cracks and burns
- Reduction in the number of grinding passes
- Reduction in the number of grinders needed and the associated direct and indirect costs
- Improved part roundness
- Improved part flatness
- Improved part size control
- Reduced movement of the part during grinding
- Elimination of grinding or hard machining on some surfaces
Other Needs and Objectives:
- Improved reliability of the case depth on the finished part
- Statistically repeatable part processing
- Deliberate placement of stresses to improve fatigue life
- Masking of areas that need to be quench-ed slower than the rest of the part
The Driving Data
Statistical process controls and sophisticated part cost-analysis software have provided a clearer picture of where the costs, time and dimensional errors occur and with what frequency. When size-tracking data is blended with the cost of each opera-tion, processing adjustments can produce large dividends. In the example below, consider that the direct-part costs, up to the assembly operations, are distributed as shown in Columns 1 and 2 of Table 2.
In the example in Table 2, although machining time was increased, scrap was decreased. Increasing grinding capacity 50% is a great way to increase capacity without a prorated increase in investment. The cost value of the reduced "start to finish" time for manufacturing as a result of the reduced scrap and shorter grinding times can also provide additional financial and sales benefits.
Why Quench Presses are Sometimes Needed
Understanding Where the Distortions Come From and How to Minimize their Effects on your Parts
There is nothing quite like austenitizing steel to let all the gremlins out of the bag. Many prudent things can be done to improve the quality of parts leaving the heat-treating process regardless of whether the parts are free quenched, quench pressed or free quenched and subsequently fixture tempered. Due to the high accuracy of parts that quench pressing can produce, these "upstream sins" are more noticeable. Without a quench-press operation, the needle is often lost in the haystack.
Quench pressing provides a part to the grinding operation with reduced grinding stock. Upstream processes that have the potential to create distortion in the heat-treating operation need to be controlled in order to achieve the full potential of a quench-press investment. Some of the major gains in quench pressing have come from appropriately addressing the manufacturing processes that produce distortions.
Minimizing Thermal Ratcheting and Nonsymmetric Strain Energy
"Thermal ratcheting" is a term coined to describe the irreversible plastic deformation of a material that can occur as a consequence of thermal expansion or contraction. For cylindrical steel parts, this typically happens when one side or face of the part gets heated or quenched much faster than the opposing surface. This plastic deformation not only damages the geometry of machined parts, it also typically leaves strain energy in the part that may reveal itself in sub-sequent operations.
If the raw material is a rolled forging, drop hammer forging or has come from a rolling mill, tube mill or bar mill, the material will invariably have "strain energy" stored in it. Annealing or normalizing operations, which are primarily in-tended to produce a certain metallurgical structure, allow most of this energy to relax if the parts are properly supported. Examples of processing that can cause thermal ratcheting and asymmetric strain energy are as follows:
Raw-material thermal treatment – Uneven cooling of post-annealed forgings can create impressive strain energy and ther-mal ratcheting.
Carburizing – When some of a tightly packed load of cold parts are very close to the vertical radiant tubes, the integrity of the part dimensions is compromised.
Quenching after carburizing – When a load is pulled out of the hot zone and dropped into the integral quench tank with noz-zles blasting the oil onto the parts from the sides of the tank, thermal ratcheting can definitely occur.
Reheat for hardening – When cold parts are placed into a hardening furnace for quench pressing, consideration of the conductive, convective and radiant heat transfer must be made. The resultant strain produced by all three heat-transfer modes determines if thermal ratcheting occurs and to what extent.
Hardening quench – Use of the incorrect quenchant in the hardening quench press can also produce thermal ratcheting and/or asymmetric strain energy that can reveal itself during tempering, cryogenic treatment or grinding.
Strain Energy From the Machining Operations
It certainly can send goose bumps down your back watching a carbide-spiked, three-jaw chuck bite into a raw forging and re-move 1/2 inch of material from the bore with a 100-HP spindle in five seconds! This kind of processing is often done oblivious to what the heat-treating and grinding operations need. Even though the ID may be gripped on a near 360-degree soft-jaw chuck for the other machining operations, the damage has already been done.
While it is almost impossible to totally avoid thermal ratcheting and strain-energy development, the heat treater should be aware of these mechanisms and to the extent possible, minimize them. Quench presses can "heal" some of these sins. Table 3 provides "before and after" results of a hypothetical 8-inch OD x 2-inch-wide x 0.25-inch wall-section cylindrical part proc-essed with and without considerations to minimize thermal ratcheting.
Additional Guidelines for Optimizing Dimensional Results
Keep the Surface Finish of the Machined Part Low
Try to avoid machining the parts with such a high surface finish that it causes more than 1/10th variance in the measurement resolution that you are using to gauge the parts. As an example, if you are going to record the OD of a part to the resolution of 0.0010", the surface finish should not cause more than a 0.0001" variability in the reading. When the surface-finish measure-ment is more, the statistical meaning of the data becomes cloudy and less meaningful.
Try to be Uniform in your Metrology Between Operations
If you are measuring and documenting the ID of the part with a two-point gauge in the grinding operation and then change to a three-point OD gauge measurement after quench pressing, you are never going to see a very clear picture. If you are going to make the investment in capital and man power, try to use the exact same gauging in the machining and heat-treating areas and have both of these emulate the critical reference surface measurements that the grinding operations use.
Avoid Handling Damage to the Parts
Nicks, scratches and dents received by the parts during handling and chucking operations can limit the quality of the parts leaving the quench-press operation if they are in the tooling contact surfaces. Even if they are not on the tooling contact sur-faces, they can still cause problems, including cracks and burns when they are ground.
Preserve the Dimensional Quality Achieved During Quench Pressing
Once the part has achieved the dimensional quality from a quench press, it is important to note that "it is not over yet," Part warpage can occur both during the subsequent tempering and cryogenic operations, so care should be exercised to assure that the parts are not dumped into baskets but are individually supported and mostly free of any external loads. The heating and cooling should be applied uniformly to the part to minimize the development of any asymmetric strain energy.
Quench Presses Designed Specifically for the Needs of the Parts
Ideal results can be obtained by custom designing the quench presses and tooling to meet the needs of the parts. It is not always realistic to expect a single quench press to ideally process parts that need 30 different designs of machining and grinding equipment. If it were possible to design a single piece of machinery to machine all the parts in an optimum fash-ion, it would invariably cost too much and be extremely complicated. To some extent, this holds true for quench presses.
The process of designing quench presses to meet part-specific needs has led to the development of a number of different types of quench presses as follows.
- Low-force presses – Relatively low tooling forces are exerted on the part allowing it to shrink during thermal contraction, then briefly grow during the transformation from austenite, and then continue shrinking as the thermal contraction continues towards the end of the cycle. This low-force concept achieves roundness and flatness while the part is hot and elastic and concurrently maintains a biasing force on the diameters of the critical surfaces to influence size.
- High-force presses for segmented dies – When yielding of the part during quenching is not a concern and the part needs a tightly controlled ID or OD, expanding or contracting segmented dies can be designed to make contact with the part imme-diately after placement into the quench press, allowing it to be rounded up and flattened while it is still weak and flexible. Bi-asing forces are applied to maintain the roundness and flatness until the target diameter is achieved, whereupon the tooling "locks up" and holds that diameter. Then when the part transforms and typically grows, the tooling follows its expansion and continues to exert a biasing force to maintain roundness.
- Form and quench presses – Parts like Belleville springs and leaf springs are sometimes best formed and quenched in the same operation. This not only eliminates the upstream forming process, it frequently makes prequench-press part handling easier and cheaper.
- Family of parts presses – Parts that share common needs and general geometries can be placed into a "family of parts" group that runs across a common tooling set, therein meeting the specific needs of the parts while eliminating the need to change tooling.
- Quench-press modules – Some manufacturers have such a diverse range of parts that it would be impractical to have a single quench press that could ideally meet the specific needs of all the parts. These "quick-connect" modules are an entire press with integrated tooling that has been designed to optimize the quench pressing of the part or group of parts. When it is time to run an-other part configuration, you simply swap in another module. All the setup adjustments on each module stay undisturbed so the next use of the module will be "plug in and play" without the need to requalify the setup.
- Accurate force control – In many cases, the influence of the tooling forces applied to the part can have a large impact on the part's dimensions. When this is true, it is vital that the force application system be designed to have a low amount of stiction in order to assure the accuracy and repeatability of the force applied to the part.
- Monitoring part size during the quenching operation – For those circumstances where the depth of engagement of the tool-ing is a measure of a part's critical dimension, displacement transducers attached to the tooling can be used to gauge the part's size before, during and after quenching.
- Self-contained quenchant systems – Such systems not only serve as a reservoir for the quenchant but can include subsys-tems that heat, cool and filter the quenchant, provide closed-loop flow control to each of the points of connection on the quench press, and capture and reclaim oil vapor from the quenching operation.
- Integrated part transfer from the quench press – In order to maximize the quantity of parts that can be quenched per hour, it is frequently beneficial to have mechanisms built into the quench press to allow it to unload the part while the robot is re-trieving the next part to be quenched.
Optimize Part Quality and Reliability with a Quench-Press-System Approach
The operator, control system, austenitizing process, handling of the part from the austenitizing process to the quench press, quenchant system, quench press and tooling all play vital roles in the final quality of the part. Ideally, when all of these items are designed by one supplier that has a solid handle on how each of these items impact the ultimate quality of the part, the end user will usually be able to reach a higher level of part quality. Such an approach can offer benefits like those listed below.
1) A common "recipe" file for processing the parts, which allows the common controller to automatically set up processing variables in the quench presses, furnace, robot and quenching system.
2) Algorithm-driven timing and sequencing of the parts through the entire system to help assure that the parts are processed the same from one production lot to the next. This can also help assure that the austenitizing time is highly consistent from part-to-part and lot-to-lot.
3) A common part-tracking program to monitor compliance with the recipe and processing rules.
4) Process data logging in common files.
This can include:
a. Part number
b. Production lot number
c. Date and time
d. Furnace temperatures
e. Atmosphere levels
f. Actual total furnace time
g. Robot grip forces
h. Transfer time between the austenitizing process and contact with the quenchant
i. Quenchant temperatures
j. Quenchant flow rates
k. Quench time
l. Quench-press clamping forces
m. Tooling position at the beginning and end of the quench-press cycle
n. Part residual temperature after quenching
o. Time between the quench and wash
p. Time between the quench and the tempering process
q. Alarms occurring during the processing r. Operator adjustments of the recipe values
5) A common connection and lockout point for utilities.
6) A single operator responsible for the whole process.
7) A single process controller and a single Man-Machine Interface (MMI).
8) A robot that transfers the part(s) in a highly consistent manner from shift-to-shift and day after day.
9) A robot that places the part directly inside the quench press instead of outside of it so the pre-quenching of the part prior to the application of the quenchant is reduced. Frequently, this pre-quenching is not uniform around the diameter of the part and has negative consequences on roundness and flatness.
Additional Quench-Press Benefits
- Processing more than one part per cycle – As manufacturers wrestle with the cost justifications for quench-press systems, they frequently end up spending more money to achieve the ability to process more than one part per cycle. As an example, a system to process two parts per cycle may only cost 35% more than the one that processes one part per cycle. Systems producing up to six parts per cycle are in operation today (Fig. 3) that can actually handle and quench six different parts at once. Bearing manufacturers may want to process the inner and outer raceways at the same time, and transmission manu-facturers may want to process the synchronizer rings and hubs at the same time.
- Using a skid-type chassis to support all the primary pieces of a system – On many systems, it is possible to mount all the separate pieces of equipment that make up a system on a skid chassis. The skid chassis can also serve as a routing sys-tem for the utilities going to each piece of equipment. This makes the installation of the system much cheaper, preserves the equipment alignments, improves overhead accessibility by eliminating utility drops and facilitates relocation of the sys-tem at a future date. Fig. 4 shows a system using a skid chassis.
- Integrated supply and exit conveyors – The addition of supply and exit conveyors to a system allows it to run while the op-erator addresses other responsibilities like gauging, making adjustments to optimize the part dimensions, preventative main-tenance tasks and production reports.
- Integrated washing and tempering – Often the incremental cost to integrate a washing and tempering operation can pay dividends by eliminating multiple handling operations. Also, for those products that have time constraints between the quenching and tempering operations, an integrated temper can help reduce the risk of exceeding these limits, especially during shift changes.
- Part temperature stabilizing for presentation to grinding – If washing and tempering are integrated into a system, the last step would be to stabilize the part temperature to whatever the grinding operation needed. This is very advantageous if you are setting up a manufacturing cell where the turning, quench pressing and grinding are all part of the cell.
- Quick, safe and logical recovery from power outages or other process disturbances – As the automation and integration has become more sophisticated, major advances in the control systems' ability to easily and quickly resume normal operations after power outages and other process disturbances have needed to be developed. This can be a substantial portion of the entire control program and its expense.
- Trapping discrepant parts – When a process disruption causes a part to exceed any of the limits established by the manu-facturer, the control system should effectively assist the operator in identifying the parts and removing them from the system so it will not get mixed up with the normally processed parts.
Addressing the existing processing of the raw material and parts may satisfactorily reduce the distortion problems to the point that a quench-press operation is not needed. If that is not the case, however, achieving the optimum part results and the best investment payback with quench-press technology often includes other considerations, some of which are listed below:
- Plan on buying a custom-designed system.
- Plan on sticker shock.
- Plan on a lead time of two or more years.
- Plan on accurately defining to your system designer what problems are you need to overcome.
- Plan on providing your system designer prints of the parts, annual production volumes and other manufacturing data.
- Do not plan on putting a sophisticated quench-press system in an environment where you would not be comfortable putting a new CNC grinder.
- Plan on needing to change your part drawings to reflect the grind-stock reductions once you have made your process changes and the process has stabilized.
- Plan on needing an operator skill set in-line with a CNC machining-center operator.
- Plan on needing highly skilled maintenance personnel that undergo 2+ weeks of training.
- Plan on working with your system designer to determine "how good is good enough" so you don't buy sophistication that is not financially defensible.
- Plan on the possibility of needing to make machining-path edits to keep the burrs out of the die contact areas.
- Plan on the possibility of needing to make machining-sequence edits to assure that the die contact areas are machined in the same chucking as your critical areas.
- Plan on spending 1-3 months working with your system designer to optimize the concept of your system before you ever get an estimated cost and delivery.
- If your parts are carburized and your grind-stock reductions are significant, plan on needing to reduce your carburizing time.
- If your part is not a popular geometry, plan on investing in prototype "proof of concept" development work
- Plan on creating a "grind-stock reduction team" that includes the machining, heat-treating and grinding operators. You may want to empower them to manage "prototype" runs to develop the "cause and effect" relationships and techniques for mak-ing the reductions. This kind of approach can substantially reduce the initial need to make multiple changes to your prints and secure all the approvals. Once the grind-stock reduction "recipe" has been developed, changes to the prints will be much easier and will involve fewer iterations. IH
For more information: Jay Duncan is president, consultant, and designer, Quench Press Specialists, Inc, 4159 S. Church St. Ext., Roebuck, SC 29376; tel: 864-576-3502, ext. 2; fax 864-576-3513; e-mail: firstname.lastname@example.org; web: www.quenchpress.com
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: quench press, fixturing, carburizing, finite element analysis, plastic deformation, austenitizing, strain energy
SIDEBAR: A hypothetical process- and cost-optimization example
1. Refine your raw-material specifications to minimize the strain energy as discussed in the article section titled "Why Quench Presses are Sometimes Needed."
2. Spend 25% more time in the machining operations:
- Eliminate yielding of the material from excessive chucking stresses to present a more dimensionally accurate and stable part to the heat-treat operation. This allows heat treating to better monitor and control its delivered product to grind due to improved symmetry in the parts, less strain "spring" in the parts and less dimensional error due to elimination of part yielding from the chucking operations. The time spent grinding out these machining errors, as well as some of the time spent dealing with movement and size changes of the part due to asymmetric removal of stock, will be reduced. Also reduced will be the number of "roughing" passes needed in the grinding operation.
- Slow down feed rates so the surface finish promotes accurate gauging in machining, heat treating and grinding.
- Assure that the quench-press tooling contact surfaces are established in the same chucking as the most critical surfaces so chucking errors will have a much smaller impact on the quenched part. Gauged datum surfaces should be created in the same chucking of the part as the critical surfaces.
- Sequence the machining paths to assure that burrs are not left on the quench-press tooling surfaces, where they can introduce roundness, flatness and size errors.
- Machine the part to compensate for statistically repeatable dimensional changes occurring in the part during heat treatment so less time can be focused on resisting these changes. This allows heat treat to focus on producing a part that is so round, flat and close to the final size that the number of grinding passes are significantly reduced, which allows the same grinding operator, grinding machinery, floor space and utilities to produce a much higher volume of parts. In some cases, you may be able to completely eliminate finishing operations on some noncritical surfaces that currently need to be finished to assure that the inertial center closely matches the geometrical center.
3. Invest in electronic gauging for the heat-treating process like the gauging that is used in the machining and grinding operations. Without it the heat treater will have a tough time developing the "cause and effect" relationships that can empower positive evolutions and reduce costs to manufacture.
4. Invest in a quench-press system for those parts that still have excessive grinding times due to distortion and size changes.
5. With the new heat-treating metrology data, determine where there is excessive grinding stock and change the machining dimensions accordingly.