Melonite Processing

The MELONITE® Nitrocarburizing Process

MELONITE is a thermochemical treatment for improving surface properties of metal parts. It exhibits predictable and repeatable results in the treating of low and medium carbon steels, alloy steels, stainless and austenitic steels, tool and die steels, cast and sintered iron.

Melonite Processing : Salt™ Bath Nitriding System

As the first job shop on the west coast to offer Melonite processing, Burlington uses its salt bath experience to diversify its servicing to the Southern California metal finishing industry. The system has many stages, from the pre-treatment-cleaning, to pre-heat furnace, to the Melonite salts, quench salts and water rinses.

Melonite Processing: Melonite QPQ

Melonite™ and Melonite QPQ™ are thermochemical processes intended for the case hardening of iron based metals. These processes are categorized as molten salt bath ferritic nitrocarburizing. During these processes, nitrogen, carbon, and small amounts of oxygen are diffused into the surface of the steel, creating an epsilon iron nitride layer (e – Fe_xN).

A degraded form of this nitride layer (gamma prime: g‘ – Fe₄N) is obtained during plasma or gas nitriding. The nitride layer is composed of two principle zones. Zone 1, called the compound or “white” layer, extends to a case depth of ~0.0004″ to 0.0008″. The compound layer is porous, which lends to the lubricity of the finish, and hard (~700HV to 1600HV). Zone 2, called the diffusion zone, extends to a case depth of ~.004″ to 0.008″.

In addition, small quantities of substrate carbon are pulled from deeper within the substrate toward the surface. The diffusion zone demonstrates a decreasing gradient concentration of carbon and particularly nitrogen as the gradient extends deeper into the surface of the substrate. This property yields a tough outer surface or shell, yet alloys the material to retain ductility, thereby lending to the overall strength of the material.

Resulting properties from these chemical and structural composition changes are increased surface hardness, lower coefficient of friction, enhanced surface lubricity, improved running wear performance, increased sliding wear resistance, and enhanced corrosion resistance. Naturally, the alloy of the substrate will influence which properties are principally affected and to what extent they are affected. The following chart demonstrates what properties are best enhanced by varying the Melonite process:

Melonite Processing: Melonite Q

• Improved Wear Resistance
• Improved Running Properties
• Increased Fatigue and Rolling Fatigue Strengths
• Heat Resistance
• Black Color

Melonite Processing: Melonite QP

• lncludes the properties of Melonite Q
• Lower coefficient of Friction
• Decreased surface roughness

Melonite Processing: Melonite QPQ

• lncludes the properties of Melonite Q and QP
• Low Light Reflection
• Further Decreased Coefficient of Friction
• Enhanced Corrosion Resistance (Not suitable for stainless)

Wear Resistance for Steel Parts and Wear Resistant Coatings

The MELONITE® Process Improves Component Wear and Hardness of Steel Properties

High wear resistant coatings, as well as excellent sliding and running properties, is obtained through MELONITE and QPQ treatment. The service life of steel tools and parts is extended. Corrosion resistance of unalloyed and low alloyed steels is greatly improved.

The MELONITE and QPQ process increases fatigue strength about 100% on notched components made from unalloyed steel parts and about 30-80% on parts made of alloyed steels. The hardness is maintained up to about 930°F and extends the surface life of steel tools and components exposed to heat.

Wear Resistant Coatings With Economic Advantages

Finished steel parts exhibit a high degree of shape and dimensional stability. Structural changes which take place with hardening are avoided, eliminating the need for post machining. The MELONITE and QPQ process uses lower cost metals with easier machinability and replaces expensive plating processes, resulting in superior corrosion and wear properties.

Diffuses Nitrogen and Carbon into the Surface

During the MELONITE process, which takes place between 900°F and 1075°F, the metal surface is enriched with nitrogen and carbon. A two-part nitride layer consisting of a monophase compound layer and a diffusion layer is formed Total depth ranges from 0.008-0.040″, depending on the composition of the base material and treating time. Hardness in the compound layer ranges from approximately HV 700 on alloyed steels to about HV 1600 on high chromium steels.

(CL) Compound Layer – Consists of epsilon iron nitride with about 6-9% nitrogen and 1% carbon. The thickness for most applications is around 0.0004-0.0008″. It improves:

• Corrosion Resistance
• Scuffing Resistance
• Hot Strength
• Wear Resistance
• Running Behavior

(DL) Diffusion Layer – Contains nitrogen, either dissolved in the iron lattice and/or precipitated as very fine nitrides. Low alloy steels give thicker layers with lower hardness. Higher alloys give greater hardness with thinner layers. It improves:

• Rotating Fatigue Strength
• Pressure Loadability
• Rolling Fatigue Strength

Melonite and QPQ greatly improves the wear properties of thin-section stampings without distortion.

Improvement of Tribological Properties Through Nitrocarburizing

Structure, Hardness and Depth of the Nitrocarburized Layer

During nitrocarburizing, a two-part surface layer is formed, initially an outer compound layer, followed by a diffusion layer below it. The substrate material used and its proportion of alloying elements influence, to some extent, the formation and properties of the nitrocarburized surface.

Compound Layer

The nitrogen-rich inter-metallic compound layer mainly contains iron-carbonitrides and, depending on the type and proportion of alloying elements in the base material, special nitrides.

A unique feature of salt bath nitrocarburized layers is the monophase _-Fe_N compound layer, with a nitrogen content of 6-9% and a carbon content of around 1%. Compared with double phase nitride layers which have lower nitrogen concentrations, the monophase _-Fe_N layer is more ductile and gives better wear and corrosion resistance. In metallographic analysis the compound layer is clearly definable fron the diffusion layer as a lightly etched layer. A porous area develops in the outer zone of the compound layer. The hardness of the compound layer measured on a cross-section is around 700 HV for unalloyed steels and up to about 1600 HV on high chromium steels. Treatment durations of 1-2 hours usually yield compound layers about 10-20 _m thick (0.0004 – 0.0008″). The higher the alloy content, the thinner the layer for the same treatment cycle. Fig. 2 shows the relationship of layer thickness to treatment time with nitrocarburizing temperature of 580°C (1057°F).

Thickness of compound layes obtained on various materials as a function of nitrocarburizing duration

Diffusion Layer

The nitrogen penetration into the diffusion layer provides for improved fatigue strength. Depending on the initial structure and composition of the core material, the nitrogen in the diffusion layer is dissolved in the iron lattice and/or precipitated as very fine nitrides.

Influence of chromium on diffusion layer hardness and total nitration depth in various 0.40-0.45% carbon steels

With unalloyed steels, the nitrogen is dissolved in the iron lattice. Due to the diminishing solubility of nitrogen in iron during slow cooling, _’-Fe4N nitrides are precipitated in the outer region of the diffusion layer, some in form of needles, which are visible in the structure under the microscope. If cooling is done quickly, the nitrogen remains in super-saturated solution. With alloyed steels which contain nitride-forming elements, the formation of stable nitrides or carbonitrides takes place in the diffusion layer independent of the cooling speed. With increasing alloy content of the steel, the diffusion layer is thinner for identical nitrocarburizing parameters. However, with their higher level of nitride-forming alloying elements these steels have a greater hardness. Fig. 3 illustrates the influence of chromium on the hardness and depth of the diffusion layer in steels with a carbon content of 0.40 – 0.45% after 90 minutes treatment at 580°C (1075°F). Total nitrocarburizing depth shown in Fig. 4 is the distance to the point where the hardness of the nitride layer is equal to the core hardness. After a 90 minute treatment the total nitrided depth is about 1.0 mm (0.040″) on unalloyed steel, but barely 0.2 mm (0.008″) on a 12% Cr steel. (See Fig. 4.)

Total nitrided depth on various materials resulting from nitrocarburizing

Fig. 7 shows the coefficient of friction both under dry conditions and after lubrication with SAE 30 oil, measured by an Amsler machine. All samples were lapped to a roughness of R_ = 1_m after their respective surface treatments and before testing. Without lubrication the nitrocarburized QP had the lowest coefficient of friction, being less than half of that of the hard chrome or case hardened surfaces. The lowest friction level occurred when nitrocarburized QPQ is lubricated. It is 3-4 times lower than that achieved with the chrome or martensitic surfaces.

Coefficient of friction values for various surface layers, with and without lubrication.
SNC = salt bath nitrocarburized

These results show the direct effect of increased oxidation as it relates to friction on the surface of the nitrocarburized samples. The QPQ sample, with its extra post-oxidation step, has a much higher friction value than the QP specimen, which had part of its original oxidation in the compound layer removed by lapping. However, with this variant, due to the fine microporosity in the QPQ sample which causes the lubrication to adhere better to the surface, this option gives the lowest friction value.

If a uniform running behavior is required the QP process is appropriate. Lubrication has only a slight influence on the coefficient of friction because the oxide layer of the outer surface was removed during the polishing operation.

It has been determined that, unlike with chrome surfaces, the coefficient of friction of nitrocarburized QP and QPQ treated surfaces remains constant, even at varying sliding speeds.

The intermetallic stricture of the compound layer, which contains epsilon iron nitride formed during nitrocarburizing, is extremely resistant to adhesive wear and scuffing. Fig. 8 shows the scuffing loads of gears made from various materials (6). It was established by applying increasing pressure to the flank tooth until galling occurred. Austenitic steel containing 18% chromium and 8% nickel had the lowest resistance to galling, however, after nitrocarburizing its resistance was raised almost five-fold. The performance with SAE 5134 was about tripled. Even SAE 5116, which had already been carburized, more than doubled the scuffing load it could withstand through the compound layer built by the nitrocarburizing treatment.

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Scuffing load limit of gears.
SNC = salt bath nitrocarburized