Make Your Atmosphere Furnace Work for You: Tips of the Trade for Carburizing and Quenching

By Rene Alquicer, Manager - Atmosphere Products; Aymeric Goldsteinas, Product Development Manager; and Craig Moller, Chief Engineer

Throughout the manufacturing process, heat treatment is consistently viewed as a critical step for adding value to the parts produced. A part expensively manufactured by melting, hot rolling or forging, annealing, rough machining, teeth cutting and grinding is essentially useless and of little to no value without heat treatment. In addition, without reliable and repeatable heat treatment, it is impossible to achieve competitive overall manufacturing costs.

Amazingly, the cost for a manufacturing step that adds such a high value is only a fraction of the total production costs - generally in the range of no more than 5%. This percentage, however, increases to roughly 15% of the costs per part if all further post-treatment process steps inherent with, or caused by, heat treatment - such as cleaning, blasting, straightening and/or grinding - are taken into account. Therefore, a noticeable reduction of the manufacturing costs is only possible by minimizing the distortion of parts. For this, all the influencing parameters like steel melting, forming of the parts, uniformity of microstructure and hardenability, as well as the natural factors of the heat treatment process - positioning of the parts in the load, uniformity of heating, carburizing and heat extraction during quenching - need to be analyzed and optimized if one is to continually produce quality parts in batch atmosphere furnaces.

When producing quality parts, we are primarily concerned with properly utilizing the carburizing and quenching processes and applicable modern technology. From optimizing controls and quenching systems to the benefits of establishing temperature and gassing uniformity, here are some tips highlighting how to make the most of your processes and atmosphere furnace.

Optimizing the Carburizing Process

While there are currently two industrial carburizing processes - gas carburizing in atmosphere furnaces and low pressure carburizing in vacuum furnaces - both have the same aim: to carburize all workpiece elements in a load uniformly, to the same surface carbon (C) content and to the same case depth. Atmosphere carburizing runs a series of different process steps. Knowledge of these steps is necessary for achieving repeatable work and uniform carburizing:

  1. Gas Reactions: generation of the carburizing gas components in the atmosphere
  2. Convective gassing: transport of the carbon-containing molecules in the gaseous phase to the component
  3. Diffusion transport: transport of the carbon-containing molecules through the boundary layer (v=0) at the component surface
  4. Dissociation and adsorption: dissociation of the molecules at the component surface
  5. Absorption: taking up of the carbon by the component surfac
  6. Diffusion: transport of the carbon into the component

Overall, the gas reactions which take place in the carburizing atmosphere are many and varied. Taking into consideration the six process steps listed above, ideal carburizing conditions exist if temperature and gassing uniformity, flow over the components and fast reaction kinetics occur evenly throughout the treatment chamber. By achieving ideal conditions in these areas, one can positively influence the quality of components produced.

Temperature Uniformity

A uniform temperature is one of the first, and most essential, steps for ensuring parts emerge with an ideal carburizing depth - and thus higher quality. In efficient batch atmosphere furnaces, such as the Ipsen ATLAS, temperature uniformity of at least ±13 F (±7 C) is maintained in the heat chamber. Upon completion of the heating phases, all components in the austenitic phase are at the same temperature.

Efficient burners - like Ipsen's Recon® Burners - can enhance the heating of batch furnaces. These burners are single-ended recuperative tubes (SERT) fitted with special ceramic inner tubes. The burners increase thermal efficiency up to 75% - simply by recovering the heat from the exhaust gases and reducing time to recovery of the hot zone temperature. Distinguished by low noise levels, high durability, low maintenance and easy installation, these modern burners provide ideal heating while optimizing gas consumption. Reliability comes from excellent furnace designs. One example of this is, if one of the burners is down in the middle of a cycle, the other burners are adaptive and will compensate so that excellent uniformity can still be achieved throughout.

However, it's important to realize that achievement of temperature uniformity is not possible without improved gas flow over the components. You must have a well-planned circulation system to achieve excellent flow around components and, thus, maintain temperature uniformity.

Gassing Uniformity

The positive convective properties, which result in excellent temperature uniformity within the load area, also result in improved heat transfer to the load and homogenous process atmosphere. Figure 1 portrays how the combination of minimum temperature deviation within the load area and the homogenous process atmosphere results in minimized case depth deviations throughout the load.

Fig. 1: Representation of the fluid flow lines in a treatment chamber.

Continuous introduction of the carrier gas and controlled additions of the enriching gas result in a furnace atmosphere capable of producing carburized parts with the specified percent of surface Carbon and to a specified case depth with highly repeatable results, as shown in Figure 2.

Fig. 2: A furnace atmosphere during gas carburizing.

Intuitive control software, such as Ipsen's Carb-o-Prof® system, can assist in maintaining balance by regulating, documenting and archiving the carburizing processes in atmosphere furnaces. No matter the case - if your power goes out or some other unforeseen event occurs - the software is able to adapt the process to the changing circumstances, preventing the potential waste of parts and resources.  Even before actually processing the load, users are able to generate a potential recipe and immediately review the process results using the advanced simulation software.

Specifically, the software monitors and controls uniformity of the C-level within the atmosphere; which, through supervision, maintains a tolerance of ±0.05% C for the workpiece surface carbon content. This consistency of the atmosphere's carburizing effect results in uniform carburizing of the surface layer, as shown in the example of a gear wheel in Figure 3.

Fig. 3: Uniform surface carburizing of a gear wheel.

When striving towards uniform carburizing, it is important to remember that temperature and gassing uniformities are interrelated - it is difficult to meet one parameter without influencing the other.

Carb-o-Prof®- Your Very Own Electronic Metallurgist

Ipsen Carb-o-Prof combines more than six decades of knowledge and expertise in a single controls system, and it is specially designed for the computation and execution of the complete carburizing and quenching cycles, as well as other heat treatment processes. Overall, it provides the flexibility needed to measure and analyze your equipment and processes with ease. You can then use this analysis to refine and adjust the settings and parameters of your equipment to enhance your process, thus improving the quality of your parts.

This unique, reliable software consists of flexible controls and straightforward user prompts and color menus - making for a user-friendly experience. Other features include an extensive recipe database, an adaptive C-profile control and a time- and cost-saving simulation function.

Recipe Database

Programmed with hundreds of available recipes, the database allows the most important recipe information to be registered via a quick, simple input. Faulty inputs are prevented by appropriately limiting the input range, thus maintaining a safe operation and avoiding excessive consumption. As a result, recipes are generated in an easy, consistent manner that focuses on the carburizing/hardness results and prevents input errors.   

Simulation with C-Profile Optimization

An outstanding, yet standard, feature of the Carb-o-Prof is its simulation function. Essentially, it computes the materials' expected carbon profile according to the entered parameters and displays the results, both as a table and as a straightforward graph - as demonstrated by Figure 4. The profile can then be re-evaluated and parameters adjusted, if necessary. Overall, this gives users the ability to review the process results for their specific load immediately after generating the potential recipe - all without having to do the actual run. Due to the fact that no test load needs to be run beforehand, no valuable parts, time or resources need to be wasted.

Fig. 4: Example of a simulated test run conducted using Carb-o-Prof software.

C-Profile Control

Using pre-specified target parameters, such as surface carbon content, carburizing depth and/or core carbon content, Carb-o-Prof is able to define a target carbon-content curve in the shape of a smooth S-form. As a result, parts within the same load are consistent in both case depth and hardness, and this outstanding performance is consistently repeated load after load.

Ceramic Muffles

Batch atmosphere furnaces possess several features that help achieve both proper temperature and gassing uniformity during various processes. One such feature is the ceramic muffle, which is made of silicon-carbide and can optimize the performance of all heat-treating processes. Atmosphere furnaces, like Ipsen's ATLAS, use ceramic muffles to shield the load from direct heat and to facilitate uniform temperature distribution throughout the material being hardened. 

The muffle provides a temperature uniformity that is especially important in case hardening because it controls the depth of penetration and carbon concentration. In addition, the muffles' ability to provide optimal circulation and a constant directional flow assists in evenly soaking the load. Beyond temperature and gassing uniformity, the next critical step is optimizing the quenching process.

Optimizing the Quenching Process

Older quench systems for batch atmosphere furnaces used to possess little flexibility with respect to varying the quench intensity. Experience shows there is significant potential for optimizing, as well as producing, a uniform quench in oil. The implementation of these techniques has produced a more uniform hardening of parts - especially of gear components - with an improved microstructure and reduced distortion.

Today's requests of adapting the quenching intensity of quench systems to the needs of different components - specifically hardenability and minimization of distortion - have also led to the increased production of quality components.

Oil Quench

Modern oil quenching systems, like Ipsen's SuperQuench®, have an all-encompassing agitation system, allowing them to produce a uniform oil flow through the load section, as well as utilize an adjustable oil flow speed. In addition, by utilizing the agitators' timing control, SuperQuench's cooling curve can reach results closer to the ideal cooling curve, as seen in Figure 5. This feature increases the efficiency and flexibility of oil quench systems and makes hardening of low-alloyed materials and thicker cross-sections possible.

Fig. 5: Representation of an ideal cooling curve with a high cooling rate to start and a cooling rate reduction when entering the martensitic phase.

In addition, the realization of complex quench cycles is easily done with the proper control software. A quench cycle is established within the software by assessing the section size, the type of material, load density, temperature and the type of quench oil.

Ideal Quench Speed and Heat Extraction

The goal of a uniform heat extraction over the whole surface part can only be reached with an equal flow of oil around the entire part. While this situation is possible with simple parts, it is very difficult with complex part geometries, yet still achievable with an efficient quench system.

It is important to be aware of the vapor phase, also commonly known as the Leidenfrost effect. This phenomenon occurs when a liquid - which is in near contact with a mass significantly hotter than the liquid's boiling point - forms an insulating vapor film that keeps the liquid from boiling rapidly. Then, as the vapor film randomly breaks down, the nucleate boiling phase starts and is characterized by a high cooling rate. The final stage is the convection phase.

One way to prevent the difficulties associated with the vapor phase is by using non-evaporable media like salt or gas. Their cooling rate in the upper temperature region, however, is usually not sufficient for low-alloyed or carbon steels. Therefore, the objective must be to optimize the oil quenching in such a way that it reproduces an ideal cooling curve.

Assuming the usage of a high-performance quench oil - as is normal in sealed quench furnaces today - a high oil flow velocity best achieves a high, uniform cooling rate on the entire surface area of a part. This speeds up the breakdown of the vapor film in areas of less flow, producing a more uniform, faster quench.

Higher oil velocities considerably improve the uniformity of heat extraction. Depending on the thickness of the part and the hardenability of the respective steel, the influence of the flow velocity can also result in further quality improvement of the part.

Reducing Distortion

The goals for a distortion-optimized quenching can generally be defined as follows:

  • Uniform heat extraction over the whole surface of the part
  • Uniform heat extraction on every part within one load
  • Material- and part-adapted timing to control the quench intensity

These goals are realized throughout the quenching cycle. The first part of the quenching cycle uses a maximum oil flow so that a quick vapor film breakdown occurs and a high heat extraction is realized in the nucleate boiling phase - all of which prevents the production of ferrite and perlite. The second part of the quenching cycle reduces the cooling rates to allow temperature homogenization between the surface and core before martensitic transformation starts. This equalizes thermal and transformational stresses; therefore, producing less distortion. As Figure 6 demonstrates, these modern, innovative systems allow you to achieve optimized performance and produce quality components as compared to a conventional oil bath.

Fig. 6: Comparison of SuperQuench's working principle (left) to that of a conventional oil bath (right). 

SuperQuench® - Increasing Productivity, Reducing the Hardness Spread

Regardless of whether you're quenching bulk or very dense loads, Ipsen's SuperQuench® system tackles even the most challenging alloy by reducing the spread of hardness values and increasing the overall production of parts. With larger propellers, stronger drive motors and optimized baffling, it provides an oil bath for the heat treatment of widely varying steel grades, including low-alloyed steels. In addition, each individual agitator has been allocated a separate flow control system so that the entire load is reliably, uniformly and quickly quenched - all of which contributes to an improved hardness distribution and better quality parts.

Overall, the SuperQuench system produces higher oil flows with improved speed. The system's flexibility allows the oil flow to be adapted to the section size, material and the load density to be quenched. For optimal heat treatment results, the flow velocity is not kept constant during the whole quench cycle, but segmented in as many steps as necessary. Other strengths of SuperQuench include its contributions to uniform hardening, reduced distortion and optimized quench speed and heat extraction.

Uniform Hardening of Components

Each agitator motor is equipped with a variable frequency drive, allowing the separate speed adjustment of each motor in the range of 10 to 60 Hz. Depending on the oil viscosity, running the motors up to 60 or 70 Hz is possible for a limited period of time.

As the oil flow is better restricted to the load area via baffles implanted on all four sides of the load in the oil tank, a high oil flow is maintained - even with very dense loads. This high oil flow results in uniform oil flow velocities between the top and bottom area of the load, including those in the top tray. Thus, the utilization of the SuperQuench process reduces the spread of the hardness values while also increasing the productivity of the sealed quench furnace as more trays can be used in each load. In practical applications, it is possible to achieve an increase in productivity by up to 50% (compared to a sealed quench furnace with a standard oil quench system).

Conclusion

When carburizing/through-hardening and quenching parts in a batch atmosphere furnace, it is essential to achieve uniformity of temperature and gassing, optimize the flow over components and aim for ideal quench speeds and heat extraction by utilizing various high-performance systems. All of which allows you to produce high-quality parts with reduced distortion, as well as achieve competitive, overall manufacturing costs via heat treatment.

In addition, through the use of modern technology - such as Ipsen's Carb-o-Prof and SuperQuench systems, ceramic muffles and Recon Burners - one can achieve ideal end results when using the carburizing/through-hardening or quenching processes. In the end, the enhanced design and control of such technologies allows you to positively impact your processes, as well as utilize the tips provided for producing quality parts.

For more information contact Rene Alquicer at +1-815-332-2695 or Rene.Alquicer@IpsenUSA.com, or visit www.IpsenUSA.com.