Alternatively, carbonitriding can be performed utilizing safe, proven, reliable and modern low-pressure technology in vacuum furnaces.
SECO/WARWICK has investigated low-pressure carbonitriding technology in vacuum furnaces to support heat treaters in preparations for the inevitable industrial transformation. Multiple different low-pressure carburizing and low-pressure carbonitriding test processes were per-formed on various steel grades. The aim of the test processes was to assess the differences in the samples’ metallographic results (microhardness and microstructure) under different pro-cess conditions. As a result, an optimal low-pressure carbonitriding recipe has been defined and carried out. The test samples from the process were further examined in a tribological test for abrasion resistance in laboratory conditions. Overall results have clearly indicated that for certain steel grades, low-pressure carbonitriding improves hardenability, microhardness, mi-crostructure, and abrasion resistance. Research has proven that modern vacuum furnaces with low-pressure carbonitriding technology can be considered as a viable and environmental-ly friendly alternative to atmosphere furnaces with traditional carbonitriding.
Having observed a growing interest in low-pressure carbonitriding and a constant increase in customer needs to improve the sustainability of their businesses, it was necessary to research available technological opportunities for performing low-pressure carbonitriding in vacuum furnaces.
In 2021, SECO/WARWICK entered into a partnership agreement with commercial heat-treating company Hart-TECH located in Łódź, Poland.
The agreement’s scope was to jointly research and develop thermochemical heat treatment technology for low-pressure carbonitriding in an industrial vacuum furnace. The first step was to select an array of steel grades with the potential for low-pressure carbonitriding processes, define the required project milestones, and establish process parameters for initial test heat treatment processes.
Carbonitriding itself has been an established thermochemical process for many years. Its low-pressure variation, which is based on low-pressure carburizing, has been gaining in popularity in recent years. The main principles of low-pressure (vacuum) carbonitriding are highlighted by Daniel H. Herring: “Vacuum carbonitriding is a modified carburizing process, not a form of nitriding. This modification consists of introducing ammonia into the carburizing atmosphere to add nitrogen to the carburized case as it is being produced. Carbonitriding is a surface-hardening treatment that introduces carbon and nitrogen into steel above the austenitizing temperature (Ac3). A martensitic case is achieved upon quenching. The hardness of the case is dependent on the carbon and nitrogen concentration of the case.”[1]
The main area of application of carbonitriding is the heat treatment of unalloyed steels, such as free-cutting steels, as well as machine and construction steels. The case depth obtained is shallower than for carburized parts (carbonitrided layers are in the typical range of 0,2-0,5 mm) as carbon diffusion is hindered by nitrogen and process temperatures for carbonitriding are usually lower and for a shorter amount of time. The result of the process is an increase in the mechanical properties of the parts by creating an epsilon-nitride phase that elevates case hardness and improves wear and anti-scuffing properties [2].
The most common method in industry is to perform carbonitriding in atmosphere carburizing furnaces, in either batch (e.g., sealed quench type) or continuous (e.g., mesh belt type) furnaces. A common feature is that with this solution, a flammable and explosive endothermic atmosphere is used, and the process itself results in the emission of not only carbon dioxide but also other toxic gases (CO, SOx, NOx). Moreover, atmosphere-carburizing furnaces must work with open flame curtains and afterburners, which pose a real and constant threat of fire. Vacuum furnaces, on the other hand, and low-pressure-based technologies counteract all those disadvantages, providing safer, faster, and cleaner processes aligned with current governmental and industrial expectations. Heat treatment in vacuum has been identified as a fundamental technology of the future and offers the ability to benefit from the latest achievements in the fields of materials and product development and manufacturing methods. [3] With stricter regulations aiming to limit the effects of global warming and decrease CO2 emissions, such as the UN Resolution Transforming Our World: the 2030 Agenda for Sustainable Development [4] The Paris Agreement [5] and as more and more countries [6] and companies declare either to reduce carbon footprint significantly or to be carbon neutral. It is impossible to achieve carbon-neutral thermochemical heat treatment processes such as carburizing or carbonitriding in traditional atmosphere furnaces, which is why more customers approach SECO/WARWICK to deliver environmentally friendly vacuum-based solutions for sustainable heat treatment.
Low-pressure carbonitriding test processes
The beginning of the research work was to assess differences in basic metallographic results (microhardness and microstructure) between the samples after low-pressure carburizing [LPC] and samples after low-pressure carbonitriding [LPCN]. Recipes for two heat treatment processes of low-pressure carburizing were defined and executed; one difference between the processes was the addition of gaseous ammonia in one of them to achieve nitrogen enrichment of the surface layer. The following steel-grade samples were used as reference loads in both processes:
- Construction steels for quenching and tempering: 36MnB4 and 42CrMo4
- Construction steels for carburizing: 21TiMnCr12 and 16MnCr5
- Carbon steels: S355J2 and C15
- Free cutting steels: 11SMn30 and 11SMnPb37
The charts below present the results obtained for surface hardness and effective case depth after the first test process.
The results achieved for surface hardness (Fig. 2) and effective case depth (Fig. 3) have clearly indicated that the application of a low-pressure carbonitriding process facilitates achieving higher surface hardness and thicker effective case depth, especially for free-cutting steels. In particular, with 11SMnPb37, there is a 13 % increase in effective case depth (Fig. 2) and surface hardness increase by 150 HV (Fig. 4) was observed. In the case of other steel grades, the increase was either minimal or a positive effect has not been observed at all.
Figure 2: Surface hardness comparison after LPCN/LPC process. (Source: SECO/WARWICK)
Figure 2: Surface hardness comparison after LPCN/LPC process. (Source: SECO/WARWICK)
Figure 3: Effective Case Depth comparison after LPCN/LPC process. (Source: SECO/WARWICK)
Figure 3: Effective Case Depth comparison after LPCN/LPC process. (Source: SECO/WARWICK)
Figure 4 Surface hardness comparison after LPCN/LPC process for 11SMnPb37. (Source: SECO/WARWICK)
Figure 4 Surface hardness comparison after LPCN/LPC process for 11SMnPb37. (Source: SECO/WARWICK)
Based on the information gathered, it has been determined that steel grades 11SMn30 and 11SMnPb37 will serve as base materials for the subsequent tests. Those steel grades are the most often used for carbonitriding in the industry and the results obtained after the low-pressure carburizing process confirmed the benefits of this process. Other chosen steel grades were rejected from further processing as the obtained results did not show significant improvement and those steel grades are not so commonly used in the industry.
Further research examined how altering heat treatment conditions affects results by conducting five additional processes with various temperatures and ammonia dosing. It revealed a wide range of effective temperatures for the processing of 11SMnPb37 steel and confirmed optimal ammonia dosing parameters. This information pinpointed key parameters for an efficient low-pressure carbonitriding process, ensuring favorable outcomes and economic efficiency.
After the test processes were performed, nitrogen and carbon concentrations in the case depth were measured as well. Obtained subsurface value for Nitrogen was ca. 0,3 % and total case depth was ca. 0,50 mm. Worth noticing is that after low-pressure carburizing, an increased depth of carbon penetration was observed, meaning that the addition of nitrogen facilitated carbon diffusion in the heat-treated material, allowing for a thicker case depth obtainable at the same time (Fig. 5).
Figure 5 Carbon concentration comparison. (Source: SECO/WARWICK)
Figure 5 Carbon concentration comparison. (Source: SECO/WARWICK)
Tribological tests
Tribological tests on the ball-disc friction pair were executed to assess the wear resistance properties of the disc’s surface. Standardized discs for those tests were made of 11SMnPb30 and underwent either low-pressure carburizing or low-pressure carbonitriding processes. The standardized counter sample for the discs was a ball made of 100Cr6 steel. The contact area of both materials in the pair was dry, and three tests for each pair were made under the following conditions:
- Velocity – 0,1 m/s
- Time – 1000 s
- Friction radius – 0,009 m
- Rotational speed – 106 rpm
- Number of revolutions – 17684Friction path – 1000 m
The results of the tribological tests were as follows:
- For normalized disc wear, the indicator sample after low-pressure carburizing showed a value over 50 times larger than the sample after low-pressure carbonitriding. This means that the sample subjected to LPCN exhibited significantly less wear than the sample treated with LPC. (Fig. 6).
Figure 6 Wear indicator comparison after
LPCN and LPC. (Source: SECO/WARWICK)
- Average coefficient of friction was lower for the disc after LPCN, which means that the braking force in this case was also lower (Fig. 7).
Figure 7 Average coefficient of friction comparison. (Source: SECO/WARWICK)
Figure 7 Average coefficient of friction comparison. (Source: SECO/WARWICK)
- The maximum depth of disc wear is notably lower for the LPCN treated disc, which indicates that disc after LPCN showed remarkably better wear resistance (Fig. 8).
Figure 8 Maximum depth of disc wear comparison. (Source: SECO/WARWICK)
Figure 8 Maximum depth of disc wear comparison. (Source: SECO/WARWICK)
Summary
This research has helped to define optimal process parameters for low-pressure carbonitriding in SECO/WARWICK vacuum furnaces. With the support of the highly advanced software tool Sim-VAC, the user defines recipe segments for low-pressure carbonitriding in accordance with the requested effective case depth. After the low-pressure carburizing segment is finished, the load is being cooled down to an austenitizing temperature at which a defined amount of ammonia is automatically injected into the furnace for a defined period. Afterwards, the heat-treated load is transferred under protective conditions to the quenching chamber and quenched in oil. The whole heat treatment process is done automatically in accordance with a pre-programmed recipe, ensuring full repeatability, and can be supervised remotely.
Based on performed tests and examinations, it has been confirmed that low-pressure carbonitriding performed in vacuum furnaces can be considered a viable and environmentally friendly alternative to atmosphere furnaces with traditional carbonitriding. Overall results have clearly indicated that for certain steel grades, low-pressure carbonitriding improves hardenability, microhardness, and microstructure, achieving an increase in effective case depth and abrasion resistance. Free-cutting steel grades have proven to be the most susceptible to the positive impact of low-pressure carbonitriding technology. Enhancement of metallographic results and improvement of tribological properties have been the most significant for those steel grades. Apart from that, low-pressure carbonitriding can be considered as a viable process as well for low-carbon steels, and given low-alloyed steels intended for carburizing. The tests have also confirmed that for medium-alloyed and high-alloyed steels, the use of low-pressure carbonitriding does not cause visible improvements in metallographic properties.
SECO/WARWICK and various industrial Partners have been exploring other potential process configurations for certain steel grades to further improve and optimize low-pressure carbonitriding technology. Current investigations include works on improving and adjusting nitrogen concentration within the effective case depth, as well as nitrogen diffusion profiles to meet specific industry requirements. Experience and technological expertise allow SECO/WARWICK to provide full and constant support in co-developing various low-pressure carburizing and carbonitriding processes. Confirmed technological results are guaranteed to be met with full repeatability in the future. With the constant development of vacuum-based technologies, SECO/WARWICK has been leading the way to a sustainable future for heat treatment technology.
References
[1] D. H. Herring, Vacuum Heat Treatment: Principles| Practices|Applications, BNP Media II (2012), 377
[2] D. H. Herring, Vacuum Heat Treatment: Principles| Practices|Applications, BNP Media II (2012), 377
[3] D. H. Herring, Vacuum Heat Treatment: Principles| Practices|Applications, BNP Media II (2012), 444
[4] Resolution adopted by the General Assembly on 25 September 2015, Transforming our world: the 2030 Agenda for Sustainable Development, https://www.un.org/en/development/desa/population/migration/generalassembly/docs/globalcompact/A_RES_70_1_E.pdf, access on: 15.01.2024.
[5] The Paris Agreement, https://unfccc.int/process-and-meetings/the-paris-agreement, access on: 15.01.2024
[6] Carbon Neutrality Coalition, https://carbon-neutrality.global, access on: 15.01.2024
Authors
G. Głuchowski
SECO/WARWICK
Grzegorz.Gluchowski@secowarwick.com
Dr. R. Pietrasik
HART-TECH Sp. z o.o.
M. Bazel
SECO/WARWICK
