Modern salt-bath plant

Nitrocarburizing offers an alternative to complicated coating processes and enables manufacturers to replace expensive materials with less costly ones. Salt-bath nitrocarburized steel, sintered iron and cast iron parts have very good wear, corrosion and fatigue resistance, as well as enhanced sliding proper-ties. In recent years, Houghton Durferrit has focused on three areas in an ongoing effort to improve the salt-bath nitrocarburizing process: 1) meeting environmental concerns, 2) improving process-technology and process-con-trol automation, and 3) broadening the field of application by increasing and lowering the treating temperature.

Fig 1 Schematic of the Melonite QPQ treatment cycle

Nitrocarburizing characteristics

Salt-bath nitrocarburizing offers many benefits including:


  • High-quality components - superior corrosion resistance by using oxidative cooling; consistent, repeatable results both with low and high throughputs; uniform, rapid heat transfer via the salt melt; uniform nitrocarburizing effect, even on components having narrow openings; and very good running-in wear behavior due to the formation of a pore zone.
  • Easy-to-use process - simplified precleaning and monitoring (only a few parameters, such as temperature, treatment time and bath composition in the MEL1 bath need to be monitored; simplified plant technology; and good part quality regardless of the size or make up of the load.
  • High flexibility - parts requiring different treatment times can be processed at the same time; different materials can be processed together in one load; treatment time and run-through time is very short; modular unit design allows easy matching to suit varying throughputs; use of media having different cool-ing rates (water, AB1 bath, air blast, nitrogen and vacuum); and treatment temperature range of 895 to 1165F (480 to 630C).

Figure 1 shows an overview of the standard variants of the Melonite QPQ (quench-polish-quench) treatment cycle. The salt bath consists mainly of alkali cyanate and alkali carbon-ate. In the process, alkali cyanate (the active constituent) reacts with the surface of the component forming a compound layer below the surface. The compo-sition of the salt melt changes very slowly, and the decomposition product is directly recycled in the bath to active cyanate by adding a poly-meric regenerator as needed. Because there is no change in volume, it is not necessary to bail out part of the melt when modifying the bath composition.

Fig 2 Influence of temperature and treatment time on the compound-layer thickness

Nitrocarburizing at higher temperatures

Of the three major limiting factors (temperature, treatment time and bath composition) in the salt bath, temperature plays the most important part. For example, increasing the treating tempera-ture from 1075 to 1165F (580 to 630C) produces a compound layer about twice as thick as usually obtained, at equal treatment times (figure 2).

In addition to forming a thicker compound layer, a sublayer forms on unalloyed and low-alloy steels. Depending on the cooling method used after the treatment, the sublayer consists of carbon-nitrogen bainite or carbon-nitrogen martensite, with a high percent-age of undercooled austenite. In nitrocarburizing, the transforma-tion to austenite begins at the point of highest nitrogen con-centration, which is the phase boundary between the compound layer and diffusion layer.

From this point, the austenitic intermediate layer grows into the diffusion layer during nitrocar-burizing. With increasing alloy content, the transformation temperature of the austenite changes. In steels containing over 5 % chromium, the trans-formation temperature is above 1200F (650F).

High-temperature treatment produces virtually no change in the corrosion resistance of C45 (AISI 1045) and 42CrMo4 (AISI 4140), but significantly improves the corro-sion resistance of steels having a higher chromium content. This appears to be attributed to the thicker compound layer. However, it is not possible to improve the cor-rosion resistance of austenitic steels.

In many cases, fatigue strength and wear resis-tance can be enhanced by treatment at higher tempera-tures. This allows achieving the same layer thickness in a short-er treatment time.

Fig 3 Schematic of the Melonite QPQ Tenoplus treatment cycle. The workpiece is heated to a higher temperature, then lowered to the standard temperature of 1075F.

A further variant of high-tem-perature nitrocarburizing is a two-stage process (figure 3) consisting of heating the workpiece to a higher temperature (for example, 1165F, or 630C), then lowering the temperature to 1075F (580C), which produces good results. The compound-layer thicknesses achievable on C45 at 1165 and 1075F (holding at each temperature for 30 minutes) are similar to those produced using the standard treatment at 1075F for 90 minutes. However, corrosion resistance is increased to 1000 h using the two-stage process compared with around 500 h for conventionally treated material (salt-spray test DIN 50 021, or ASTM B117).

In addition, both conventional salt-bath nitrocarburizing and two-stage nitrocarburizing are very economical. A successful salt-bath treatment can increase throughput of an existing plant by about 33%, with a consequent reduction in treating costs. Two-stage nitrocarburizing has a shorter treatment time, thereby increasing plant capacity.

Fig 4 Comparison of hardness profiles for low- and high-temperature nitrocarburized X45CrSi9.3 (AISI HNV 3) Fig 5 Comparison of hardness profiles for low- and high-temperature nitrocarburized X42Cr13 (AISI 420)

Treatment at 895F (480C)

Salt-bath nitrocar-burizing until now has been carried out at a temperature of 1075F (580C), and in special cases, at a maxi-mum temperature of 1200F (650C). Tools and components used in engine construction and mechanical equipment in the automotive industry often require a high core hardness after nitrocarburizing. As a rule, this core hardness can be achieved using temper-resistant materials. However, salt-bath nitrocarburizing sometimes still can be used when temper-resistant materials are not available.

Typically, a salt bath can only be operated at temperatures above approximately 1020F (550C) because the carbonate begins to crystallize when the bath temperature drops below this point. Adding regenerator can overcome this problem and make it possible to operate a salt bath at a temperature of around 895F. Regenerator reduces the carbonate and automatically raises the cyanate content. However, the nitriding activity of the bath is much lower at a reduced tem-perature despite the increase in cyanate content. Corre-spond-ingly, the thickness of the com-pound layer is far more shallow than the layer produced at 1075F. Figures 4-7 illustrate diffusion-layer formation by means of hardness profiles for four different materials.

X45CrSi9.3 (AISI HNV 3): A surface hardness of over 1200 HV (0.1) is obtained on ferritic-martensitic valve steel after a treatment time of 6 h (figure 4), which exceeds the hardness of the sample nitrocarburized at 1075F. How-ever, the nitrogen penetration depth of about 40 Km (0.0016 in.) after 6 hours at 895F is low com-pared with 60 Km (0.0024 in.) after 1.5 hours at 1075F.

Fig 6 Comparison of hardness profiles for low- and high-temperature nitrocarburized X155CrVMo12.1 (AISI D2) Fig 7 Comparison of hardness profiles for low- and high-temperature nitrocarburized 42CrMo4 (AISI 4140)

X42Cr13 (AISI 420) and X155CrVMo12.1 (AISI D2): The hardness profiles for these materials-both commonly used in cold-work applications- are shown in figures 5 and 6, respectively. A drop in hardness occurs in the core when nitrocar-burizing at 1075F. Virtually the same surface hardness is obtained with a treatment duration of 3 h or more at 895F as with treatment at 1075F. The depth of nitriding is not as good as that obtained from the 1075F treatment. However, one can obtain depths of 40-50 Km (0.0016-0.0020 in.) and 50-60 Km (0.0020-0.0024 in.) and the core hardness is completely retained.

42CrMo4 (AISI 4140): There are no large quan-tities of special nitride formers in this material to impede the diffusion of the nitrogen, and consequently the hardness profiles are closer to each other (figure 7). If the treating duration is long enough, good case depths can be obtained at 895F.

The treatment has no effect on core hardness for X45CrSi9.3 tem-pered at around 1275F (690C). However, the results are very different for both of the cold-work steels. Core hardness of the hardened and tempered condition (tempering temperature = 930F, or 500C) is retained after treatment at 895F, and can even increase somewhat due to the aging effect. A distinct decrease occurs using a salt-bath treatment at 1075F. A slight decrease can also occurs in Q&T 42CrMo4 (AISI 4140) steel.

In a hard-ened and tempered (at 930F) ledeburitic 12% chrome steel, despite a reduction in compound layer thickness from about 10 Km (0.0004 in.) to 2 to 3 Km (0.00008 to 0.00012 in.), the low-temperature variant has a higher surface hardness (HV 10) and core hardness. Fur-thermore, there is a distinct reduction in the precipitation of nitrides along the grain boundaries.

Researchers conducted a wear test using the Amsler method to determine whether wear protection is increased by compound layers produced at 895F. Abrasion discs made of Cf53 (AISI 1053) were treated using the two-stage nitro-carburizing (1130 and 1075F salt bath), the standard process at 1075F, and the low temperature process at 895F to produce a compound layer of about 15 Km (0.0006 in.) thick on all test pieces. The two-stage process produces the best results. There is little dif-ference between the 1075F and the 895F variants.

This test confirms the significant improvements ob-tained using the salt bath treatment when parts have to withstand adhesive wear. The untreated test pieces have a weight loss of 200 mg after only 1000 rotations. By comparison, the nitro-carburized test pieces have less than half that loss in weight after 160,000 rotations.

The low-temperature treatment extends the field of application for the salt bath process. Apart from higher surface and core hard-ness and thinner compound lay-ers, low-temperature salt bath treatment promises further bene-fits. For example, in maraging steels, the enrichment of nitrogen compensates for the reduction in the volume loss. Moreover, nitriding increases the surface hardness and wear resistance, and enhances fatigue. Low-temperature treatment also offers very good dimensional and shape stability, and it offers the possi-bil-ity of nitrocarburizing high-speed steels almost without forming a compound layer.

Fig 8 Schematic of effluent-free salt-bath plant

SIDEBAR: Progress in Salt Bath Technology

Houghton Durferrit continually is working to advance salt-bath process technology. For example, it is standard practice for salt-bath heat treatment to be carried out in automated computer-controlled plants in open and closed facilities. A new-generation automated salt-bath plant incorporates large sliding doors to permit easy access to the salt-bath units for maintenance work. With this system, replenishing salts and regenerator are added to the bath via an adjustable vibrating chute from a bin located outside the plant.

Plant parameters are computer controlled. Information is displayed on a monitor and process details are documented on comprehensive reports.

Salt melts of both automated and manually operated baths are continuously desludged via filtration systems. Cooling baths also are desludged using this method.

In the past, increased environmental legislation triggered innovations in salt-bath technology. The salt-bath process has been made environmentally friendly, and is in accordance with environmental and safe workplace regulations. Some of the most important advances of recent years include the use of regenerators to reduce waste, and the development of effluent-free heat-treating facilities (figure 8).

With regard to wastes, there is a growing opinion in the chemical and other industries that it is preferable to store a small, authorized amount of toxic industrial waste rather than process such wastes, and, thereby, produce a larger amount of less toxic waste. The less toxic waste could be discharged irresponsibly into the environment due to a lack of official regulations. Therefore, Houghton currently guarantees service for the disposal of wastes emanating from its products.

In Denmark, Germany and Switzerland, countries that have high environmental-protection standards, several manufacturers have received certification for an environment management system according to DIN ISO 14000 for using the salt-bath process. This further demonstrates the environmental compatibility of the process.