How it’s Done and Why: Transitioning Parts from Atmosphere Carburizing to Low Pressure Vacuum Carburizing
Author Vincent Lelong | Synergy Center Manager & Metallurgist, ECM USA
For more than 25 years now Low Pressure Vacuum Carburizing (LPC) has taken over several industries as the main carburizing choice. These industries take advantage of LPC’s clean environment, versatility, “just in time” processing, along with possible distortion control that comes with high pressure gas quenching (HPGQ) capabilities to process millions of parts each month. However, there are still hundreds of types of parts that can be converted to LPC with minimal effort. Below, ECM will show the recent history and products currently being processed in LPC and how they were transitioned to LPC. Metallurgical results will be shown along with production loading scenarios. In addition, the process of a particular part showing timing and cost of each treatment process will be reviewed. These facts will show the audience the benefits and how they can take advantage of low pressure vacuum carburizing.
LPC TECHNOLOGY DEVELOPMENT
Many customers committed to LPC on new product manufacturing. The challenge was to convert existing carburized parts (atmosphere furnace & oil quench) to LPC with HPGQ. The advantages of vacuum carburizing were quickly observed: better case depth uniformity, repeatability of the process load to load, and the absence of intergranular oxidation when compared to atmosphere carburizing.
HPGQ EVOLUTION
The HPGQ cell had continuous improvements to achieve metallurgical targets & increase load size, especially with turbine design and higher quench power. Pressure variation, speed and quenchant media direction are metallurgical factors & distortion controls. At the same time as improving the quench cell design, various gas quenching processes were developed to achieve higher mechanical performance. A StopQuench process was specifically designed to closely simulate the multiple stages of a heated oil quench to generate higher mechanical properties. After StopQuench was developed, not only could the oil quench performance be simulated, but it also demonstrated that distortion could be controlled and reduced with more efficiency with gas quenching media than with oil quenching.
FIXTURE DEVELOPMENT
To heat treat at higher temperature, alloy fixtures must resist deformation or cracking during heating and quenching. Therefore, they also evolved over the years. With classic pusher furnaces, carburizing is done between 920-950°C with austenitizing around 850°C. With LPC, the range of carburizing is between 960°C to 980°C and sometime higher. Fixture suppliers have improved designs by:
• Resist deformation/cracking (up to 1050°C)
• Design for higher productivity (compact load)
• Improve weight ratio part/fixture with carbon fiber composite (CFC)
• CFC-Alloy mix fixture for weight & high temperature improvement
STEEL DEVELOPMENT
For most new production with enhanced LPC + HPGQ in the last 25 years, steel production has been tasked with manufacturing material with more requirements and tighter tolerances than ever before. Steel manufacturing has developed materials:
• For HPGQ responds to higher requirements with higher DI (Jominy)
• Micro-alloys alloys (ex. Niobium or Boron) are in small quantity to allow increase in temperature with grain & hardenability control.
• These improvements allow higher load size and reduce quench parameters for distortion control.
MODULAR EVOLUTION
Most industries have started to invest in LPC installations around the world, and furnace installations have become larger to perform different treatments, such as:
• Oil / Gas Quench Cells
• Vacuum Purge Tempering
• Nitriding and Nitrocarburizing (FNC)
• Post-Oxidation
At the same time, furnace installations are being integrated into the full heat treat processes with advanced automation for quality and productivity control:
• Washing machine
• Pre-oxidation oven
• Cryogenic Treatment
• Tempers
• Robotic Load Assembly/Disassembly
When HPGQ is not the customer preference, it has become another option to add to the LPC system. With oil quenching abilities on a vacuum carburizing system, the possibilities of part type, shape and weight are almost endless for heat treatment.
LPC DEVELOPMENT STUDY
The following study is a re-evaluation of one of the multiple studies that were developed to show ease of the transition from atmosphere carburizing to LPC. Since the customer already had a vacuum furnace on site for one type of part, validation of the other part like cage was the next step of re-evaluation. In this case, core hardness was the main factor of strength performance following the benchmark atmosphere carburizing with oil quench. Fixture design was also added to improved productivity and potentially distortion control associated with gas quench study. Another factor was the size of the vacuum furnace to heat treat large/long parts. The volume of a load at 1000x610x900 mm translates into a higher throughput of cages per load. This gives an even higher potential to be more cost efficient versus the pusher or large batch atmosphere furnaces.
In 2008, with improvements to the gas quench cell and the development of Stop & Step quench technologies, the subject was revisited. The customer at this point saw varying distortion from atmosphere carburizing with oil quenching, so control of the part distortion was added to the request. The preliminary results were very promising and had potential to reduce grinding operation; however, the customer did not commit to the technology at that time.
A few years later with a new program implemented, the LPC process was looked at again and this time embraced by the company. Since they already have a vacuum furnace on site (and with the increasing age and maintenance costs of the atmosphere pusher carburizers), validation of the cage was the next step to ensure the transition. Another factor was the size of the vacuum furnace to heat treat large/long parts. The volume of a load at 1000x610x900 mm translates into a higher throughput of cages per load. This gives an even higher potential to be more cost efficient versus the pusher or large batch atmosphere furnaces.
As can be seen in this study, the same carburizing recipe and gas quench parameters from back in 2008 was used to validate the job. With all the installation improvements and a higher load volume, metallurgical results were comparable to an atmosphere pusher furnace with only a 4 to 5 HRC point core hardness difference to that of oil quench. With further analysis, the expectations of the customer have been achieved and even surpassed. Not only were the typical metallurgical results obtained, but no intergranular oxidation (IGO) was found, and the added distortion reduction requirement of the part was also met.
Another benefit seen by the company was the added control of the LPC process. The primary testing in 2007 took 5 test loads for 4 different cage part numbers to achieve the correct metallurgy. The development of StopQuench required an additional 7 tests, including 5 loads with thermocouples inside the parts, to provide uniformed cooling profiles through the load. In 2015, only 2 tests with slight modifications were needed not only to prove the performance of the LPC but also to optimize part performance. This shows that transitioning to vacuum carburizing can be a relatively easy process when a customer and supplier work together to develop and embrace new technology.
In addition to reproducing past developments, it was also possible to verify other variations to ensure productivity. The first variation was to create a larger installation. The second was to use CFC fixtures to achieve significant improvement of the part surface area versus the weight of the fixture.
SUMMARY OF RESULTS
2008 Initial Results with FIMA Turbines
Core hardness not acceptable, others metallurgy equivalent to benchmark
2015 Results with ENSAM Turbines
Core hardness slightly lower than oil results but considered optimal and comparable for all metallurgical aspects (hardness profile, microstructure)
Metallurgical results under LPC with HPGQ were considered optimal when data was compared to the current atmosphere carburizing with oil quench process, which was the initial design benchmark for heat treatment of this part. The surface hardness, core hardness, effective case depth, total case depth and microstructure were all considered.
As seen in the table above, the initial results in 2008 did not compare to the core hardness benchmark; however, all other parameters are considered equivalent. This is also seen in Graph 1, which compares all trial microhardness case depth data. Note, there is a slight difference in the transition from surface to core of the LPC and atmosphere carburized samples. Parts tested from the 2015 alloy are considered optimal since they are comparable for all metallurgical aspects, including core hardness.
The parts from the 2015 alloy fixture are also equivalent to atmosphere carburizing in that they all passed product performance requirements.
Additionally, the study of the mechanical properties and how the fatigue test on the cages used through both the LPC with gas quench and atmosphere carburizing with oil quench had very similar fracture mechanisms, morphologies and origins as shown on Figure #.
FATIGUE FRACTURE ANALYSIS
Core hardness and fatigue strength go together and benchmark trans-granular fracture is typical due to high load fatigue with final fracture from overload. The study shows that 2015 LPC + HPGQ has same fracture origin and almost fully trans-granular fracture due to high load fatigue as well.
The fracture origin occurs on the outer diameter corner microstructure around the fracture origin shows on both treatments due to high load fatigue with a final fracture from overload. Scanning electron microscopy showed the vacuum carburized fracture origin was mainly trans-granular, which was very similar to the atmosphere carburized cage. However, the atmosphere cage showed almost completely trans-granular fracture morphology at the origin, which is indicative of high load fatigue. The microstructure analysis shows:
• Tempered martensite on surface
• Core structure: Martensite + Bainite
• LPC was performed at higher temperature: some slightly coarse grain size were found
IGO generally has a negative effect on the fatigue strength of automotive parts:
• Atmosphere samples: up to 25 microns
• LPC cage microstructures: None
One rather noticeable difference in microstructure between the two types of carburizing process was the amount of intergranular oxidation (IGO) observed at the surface of the parts. There are numerous trials and papers that have shown how IGO generally has a negative effect on the fatigue strength of automotive parts. The analysis of the inner diameter micrography of the cage without etching, and at a magnification of 1000, shows the atmosphere carburized sample IGO depth up to 25 microns (Figure 9). None was observed in the low pressure carburized cage microstructures (Figure 10). This is a significant benefit of using a vacuum environment with short boosts of carburizing gas (ex. Acetylene) and diffusion boost under nitrogen instead of a continuous atmosphere of mixture of gases with oxygen content that allows for oxidation between grains.
Another success of the vacuum carburized part is the reduction in distortion from the gas quench. For this part, the roundness is the most important factor as it had to specifically fit inside another spherical part. Current production atmosphere carburized parts show a typical out of round between 0.06 and 0.09 mm. Due to this heat treat distortion, the part must have final machining to allow the correct fit into the mating component. On a limited sample size of 6 parts, the LPC parts showed an out of roundness distortion of only 0.02 to 0.03 mm. This significant improvement may allow for eliminating final grinding – especially with CFC fixtures.
LPC with a gas quench can also have major impact on post heat treatment processing. LPC has shown reduction in distortion is possible. With that reduction, final hard turning or machining to ensure specific final part dimensions may not be necessary. In many cases, this can be significant cost savings.
LPC with a gas quench can also have major impact on post heat treatment processing. For example, potential process reduction or elimination would come from reduced distortion, as mentioned in the results section. LPC has shown reduction in distortion is possible. With that reduction, final hard turning or machining to ensure specific final part dimensions may not be necessary. In many cases, this can be significant cost savings. Figure 11 shows one potential atmosphere to low pressure vacuum carburizing scenario.
Another example may include a coating process reduction. Many companies that paint parts after heat treat have complaints and hardships about baked oil or scale from traditional atmosphere carburizing. If the same part were to be processed using a low pressure vacuum carburizer with a gas quench, no cleaning process after heat treat would be needed as the part surface would be clean with no oil or scale. This not only saves money but also eliminates potential risk to the end user.
CONCLUSION
Transitioning an existing heat treat operation from atmosphere to LPC carburizing can be done easily. It is possible to show acceptable results and significant advantages when making the transition to LPC + HPGQ with minimal time and testing – not only for new products but also converting existing parts.
• Cleaner Work Environment
• No IGO
• Distortion Control
• Dense Load and Repeatability
• Metallurgical Achievement
• Mechanical Properties
• Potential Manufacturing Step Reduction
All these factors contribute to a more streamlined full view of part manufacturing. To note, in cases where the performance of oil quench cannot be matched with HPGQ, the transition is still much easier, and most of the LPC and modular installation advantages are still applicable for better results.
References
A. Welch, V. Lelong, How It’s Done and Why: Transitioning Parts from Atmosphere Carburizing to Low Pressure Vacuum Carburizing, ASM Heat Treat 2015, Detroit, USA
V. Lelong, D. Beauchesne, LPC Distortion Data Comparing Oil & HPGQ, ASM Heat Treat Conference, 2017, Columbus, USA
V. Lelong, V. Esteve, LPC – What Does it Mean to Metallurgy, ASM-Mexico, 2016, Queretaro, Mexico
D. Beauchesne, LPC with OIL & GAS Quenching, FNA 2016, USA