Metallurgical Articles

Technical Information – Arcor Nitrocarborizing

ARCOR® process

Our Process Fully Meets And Exceeds AMS 2753C Standard
“Liquid Salt Bath Ferritic Nitrocarburizing Non-Cyanide Bath”

Introduction

ARCOR® is a ferritic nitrocarburizing process in a fused cyanide free salt bath (named Sursulf®) followed by a blackening process (named “Oxynit®) to produce a surface that has exceptional resistance to wear, seizure, scuffing, fatigue and corrosion at significantly lower cost.

Metallurgy of Ferritic Nitrocarburizing

As it can be seen in the Figure 1, nitrocarburizing produces a functionalized multi layer consisting of porous and dense layers in compound zone and diffusion zone.High wear, scuffing, and corrosion resistance of nitrocarburized parts can be due to compound layer and favorable fatigue and also wear properties (if the compound layer has been removed) can be ascribed to the diffusion zone (Figure 2).The properties of these zoned has been discussed in details as follows:
Figure 1- Metallographic cross section of nitrocarburized AISI 4140 steel cross section [1]
Figure 2- Schematic cross section of nitrided region of a steel component showing the
compound layer and the diffusion zone with their (possible) constitutes [2]

Compound Zone

Structure of Compound Zone

The compound zone or “white layer” is a very hard layer that does not diffuse into the steel and remains on the immediate surface. The composition of white layer in the ARCOR® process consists of ε-phase (Fe2(N,C)1+x), predominantly. The carbon and nitrogen contents in ε-phase are variable depending on the steel grade being treated and the molten salt composition, this can vary between 0 to 8 and 15 to 33 atomic percent, respectively.

In comparison with γ’-phase (Fe4N) which usually forms in gas nitriding process and is extremely brittle, ε-phase has a higher ductility.

Porosity in Compound Zone

The outer part of the compound layer is somewhat porous. The formation of porosity is due to the decomposition of thermodynamically unstable nitrides into iron and nitrogen gas at discontinuities (grain boundaries, slag inclusions, etc.).

Presence of pores in compound layer has conflicting effects. In one view, pores result in poor adherence and low superficial hardness. In the other view, pores will form small reservoirs to hold surface lubricants to increase run-in and create a self-lubricating surface with higher wear resistance. Therefore, an optimum density of porosity in the compound layer would help to increase the wear resistance.

Increasing in nitrocarburizing time and temperature leads to increasing in porosity.

The density of porosity in low alloy steels is greater as compared to high alloyed steels. Therefore, the duration of nitrocarburizing processes in low alloy and low carbon steels is limited by the fact that porosity reaches to enough high values.

Hardness of Compound Zone

The compound layer hardness is about 700 HVN for low alloy steels and the hardness increases with increasing alloy content up to 1500 HVN in the alloyed steel as shown in figure 3. In another study the results show that increase in chromium from 0% to 17 % leads to an increased hardness compound layer from 52 RC to 70 RC.

The measured hardness falls as the degree of porosity in the outermost compound layer surface increases. The average hardness of this compound zone remote from porous portion is about 800 VHN.

Thickness of Compound Zone

The thickness of compound zone is approximately between 10 μm to 14 μm of which the first 2 μm to 4 μm contains fine pores as described above.

Alloy content in the steel has an influence on compound layer thickness, which gets thinner with increased alloy content of the steel.

Increasing alloy content of the steel means decreased compound layer thickness. Also decreased carbon content of the steel will decrease the compound layer thickness (Figure 3).
Figure 3 – Dependence of hardness and thickness of compound layer with the total alloying content (sum of Cr+Mo+V+W) in the steel [3]

Diffusion Zone

Beneath the compound zone is a deep zone is saturated by nitrogen. Alloy content is most important also for the diffusion zone hardness.

There are two mechanisms which determine the diffusion zone hardness. First mechanism is hardening due to the interstitial solution of nitrogen in steel microstructure. Similar to carbon, small amounts of nitrogen increase the hardness of steel drastically. This mechanism is more effective in low alloy steels. The second hardening mechanism in the diffusion zone is precipitation hardening due to the formation of nitrides of iron nitrides or alloying elements. For alloyed steels this hardening mechanism is predominant.

Diffusion zone thickness increases with time and temperature. For a constant time and temperature the depth of diffusion zone in high alloy steels in less than low alloy or carbon steels.

Properties of ARCOR®
Abrasive Wear Resistance

The hardness of the compound layer will determine the wear resistance. Increased hardness in the compound layer normally gives increased wear resistance. This is definitely valid for abrasive wear resistance where abrasive particles, such as sand or metal debris, wear the surface. Adhesive wear resistance of steel is dramatically improved after nitrocarburizing. This is more than what is expects from  hardness alone. 

Since the compound layer in Sursulf® process consists of just ε-phase, there is a reduction of loss of particles from the surface as compared with the amount of lost from biphasic compound layers containing both γ’-phase and ε-phase.

Case Study:

The life of punches and dies used for hot forming machines can be increased by Sursulf® process. Dies made of 0.35%C, 1%Cr, and 0.35% Mo produces 500 moulding. After surface treatment, there was no evidence of deterioration of either tool until 2000 forging had been produced [4].

Adhesive Wear Resistance (Scuffing)

The presence of a shallow porous layer at the surface of the compound layer increases the scuffing and seizure resistance properties. The compound layer gives low friction and low tendency to “cold weld” opposing steel surfaces and the porous outer zone in compound layer serves as lubricant reservoir. These entire properties act together to give excellent adhesive wear resistance or scuffing.

Case Study:

The plates were of quenched and tempered steel and in use were running against bronze in hydraulic oil. The load varied between 0 and 45 N/mm2, the speed of rotation was 1500 rpm and the surface finish of the plates 0.05 microns CLA (Centre Line Average). It was found that when using hard chromium plates with the super finished surface, scuffing occurred after 20 hours working, whereas Sursulf® treatment of the plates maintained both plates and bronze followers in good condition for 200 hours [4].

High Fatigue Resistance

The hardness and depth of the diffusion zone will determine fatigue strength. In addition to case depth and hardness also the obtained compressive residual stress state in the case is of major importance.

Fatigue strength is due to nitrogen being held in solid solution, an effect that produces strain on the ferritic lattice structure. This compression strain increases the fatigue strength.

Case Study:
Figure 4-Comparative surface fatigue tests on AISI 4137 steel and gray cast iron [4]

High Corrosion Resistance

Compared to other thermal or thermochemical surface hardening methods on steels nitrocarburizing are unique in that corrosion resistance is improved.

New developments of the process have even more improved corrosion resistance at the added benefit of a deep black aesthetically pleasant surface appearance (Oxynite® process).

This is obtained by a passivation (slight oxidation) in an oxidizing molten salt bath creating magnetite (Fe3O4) embedded in the porous layer with a thickness of less than    2 μm subsequence by impregnation of rust preventive oil coatings. 

Case Study:

The production of nitrocarburized compound layer with low porosity (without Oxynite®), can provide 300 hours resistance in salt spray on simple shape parts.

A salt spray test according ASTM B117 demonstrates that the average corrosion resistance of a ARCOR treated SAE 1035 steel shifted from 24 hours to 700 hours for simple parts and 400 hours for more complex parts. In all cases only single rust spots were visible when the parts failed, never bigger areas were affected. To compare, the resistance of hard chromium plated steels and galvanized steel against salt spray test are around 120 and 64 hours, respectively [1].

ARCOR® Applications

The applications of Sursulf® process in various industries with the typical examples has been listed in table 1.
Table 1- Typical applications of ferritic nitrocarburizing process (Sursulf®) [4]

ARCOR® Advantages

1- ARCOR® treated components possess high wear, seizure and scuffing resistance, improved fatigue strength, and enhanced corrosion resistance simultaneously. Selecting a surface modification process that can provide all of these properties is rather limited.

2- Since the ARCOR® process is carried out at a low temperature, the dimensional changes and distortion are negligible.

3- The ARCOR® process is non-toxic, and there is no environmental and disposal problems.

4- All types of ferrous materials (all types of cast irons and all types of steels including stainless steels) can be treated by ARCOR®.

5- The ARCOR® process eliminates any risk of failure due to hydrogen embrittlement which is has to be taken into account for high strength low alloy steels (HSLA). This is a major problem with many other applied coatings especially chromium plating.

ARCOR® vs. Gas Nitriding

Compare to traditional gas nitriding process that typically lasting between 5 to 100 hours the ARCOR process is done in a period of 1 to 2 hours. This short time of process causes competitive cost and lower energy consumption.

As it can be seen in Figure 5, the structure of compound layer in gas nitriding is biphasic (stacked or mixture of γ’-phase and ε-phase) or predominantly γ’-phase. This type of compound zone demonstrates a brittle superficial layer that can lead to exfoliation and delamination of nitrided layer.
Figure 5 – Schematic of different types of compound layers formed in different processes

ARCOR® vs. Plasma Nitriding

There are numerous problems encountered in technology of plasma nitriding that it is not a deal in ARCOR® process [5]. Among of different problems in plasma nitriding such as edge effect, arcing, hollow cathode effect, sputtering, and non temperature homogeneity of the work load, the first one is most important.

The edge effect (Figure 6) is a problem that happens when nitrogen penetrates into the surface from all angles of the corner and therefore the edges which consist of nitrogen saturated structure and nitride networks becomes very brittle and can easily break down. The edge problem is very common in nitriding processes with the high nitrogen potentials.
Figure 6- Schematic of the edge effect which is a phenomenon that can occur in any type of diffusion heat treatment process especially in plasma nitriding or gas nitriding [5]

ARCOR® vs. Hard Chromium Plating
ARCOR® PROCESS HAS FOUR MAJOR ADVANTAGES AGAINST HARD CHROMIUM PLATING:

1- As mentioned in the above, the wear and scuffing resistance of parts treated by the Sursulf® process is significantly higher than hard chromium plated components. Although presence of intrinsic microcracks in the chromium plating are considered as a merit to create self lubricating and good run in lubrication, the shallow porous layer in compound zone of nitrocarburizing plays such a role but with better performance.Also since the Sursulf® is a diffusion process the adhesion of hard layer to the core metal is more than the hard chromium.

2- As mentioned in the above, the corrosion resistance of nitrocarburized coatings is really higher than chromium plating. The microcracks in the chromium plating, that cause steel substrate exposes into the environment, make the corrosion problem worse by leading to galvanic corrosion with a high cathode to anode surface area ratio.

3- Utilizing and emission of carcinogenic hexavalent chromium ions as well as low cathodic efficiency (wasting energy and increasing green house gasses) are major reasons that engineers are intending to replace an environmental friendly candidate like ARCOR®. As it will be mentioned in the following, there is no toxic emission with the ARCOR® process.

4- The coating of parts with the a complex shape in chromium plating is not easy due to the low throwing power of chromium bath, while in ARCOR® process it is not considered as an issue at all.

ARCOR® vs. Carburizing

The only reason that steel parts treated by carburizing is increasing the wear resistance of parts while maintaining the toughness of core. Figure 7 shows how the wear resistance of nitrocarburized steels is superior to carburized parts [6].
Figure 7 – Adhesive wear resistance of nitrocarburized and carburized steel parts measured by pin on disc test without lubrication [6]

Cost of Nitrocarburizing

When compared to other methods of achieving the same surface benefits, ferritic nitrocarburizing is an economic one. Figure 8 presents an approximate cost comparison of various surface treatments.
Figure 8- Approximate relative costs of various surface treatments [7]

ARCOR® and Our Environment
Energy Consumption

Figure 9 shows a result of a study on the ecological impact of the various nitrocarburizing processes was published in 2011 by Bremen University (Germany). The results are favouring the benefits of ARCOR® over the other nitriding processes by taking into consideration the energy consumed, the emissions and wastes created, on an annual production of 1390 tonnes of crankshafts.
Figure 9 – A comparison between environmental and health impact of
ARCOR® and gas nitriding processes [7]

Disposal of Toxic Substances

The molten salt of Sursulf® consists of alkaline mineral salts (carbonate and cyanate) and do not contain of considerable trace of cyanide. Therefore, there is no way to be concerned by the environmental regulations in Canada.

In contrast with cyanides (CN–), cyanates (CNO–) which are used in ARCOR® are not toxic and their toxicity is 1000 times less than cyanides. One of the applications of cyanide is using as fertilizer because of high volume of nitrogen therefore it cannot be toxic.

What Do Our Customers Need to Know before Ordering ARCOR® Services?

1- Our ARCOR® process fully meets and even exceeds AMS 2753C standard “Liquid Salt Bath Ferritic Nitrocarburizing Non-Cyanide Bath”.

2- The composition of nitrocarburizing salt and Sursulf® process are monitored constantly by our in-house quality control laboratory to meet our strict quality standards.

3- Our nitrocarburizing systems can accommodate sizes up to 24″dia x 96″long and weights up to 2500 lbs. There is no limitation for small sizes.

4- In most cases parts can be turned around within 24 hours.5- In order to avoid distortion during ARCOR® process, a stress relieve treatment prior to nitrocarburizing of steel parts which were undergone high cold work and/or machining is highly recommended6- For customers who the dimensional change is very important a neutral hardening (quench and temper) treatment is recommended before ARCOR®.

5- In order to avoid distortion during ARCOR® process, a stress relieve treatment prior to nitrocarburizing of steel parts which were undergone high cold work and/or machining is highly recommended

6- For customers who the dimensional change is very important a neutral hardening (quench and temper) treatment is recommended before ARCOR®.

CONTACT US
References

[1] http://www.hefusa.net*

[2] ASM International, “Fundamentals of Nitriding and Nitrocarburizing”, Metals Handbook, 2013, P 620*

[3] T. Holm & L. Sproge, “Nitriding and Nitrocarburizing”, AGA*

[4] Source: K. H. Prabhoudev, “Handbook of Heat Treatment of Steels”, 1998, McGraw-Hill, P 288 *

[5] A. S. Biró , “Trends of Nitriding Processes” , Production Processes and Systems, vol. 6. (2013) No. 1. pp. 57-66

[6] The Linde Group, “Gas Nitriding and Nitrocarburizing”, P 17

[7] J.R. Davis, Surface Engineering for Corrosion and Wear Resistance, ASM International, 2001, p 191]

* references by a few emendments in the results.
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