Ferritic Nitrocarburizing / Nitemper

img-21This term is used to describe a large family of processes in which nitrogen and carbon are absorbed into the surface layer of a variety of carbon and steel alloys.

(Other trade names Ferritic Nitro Carburizing, FNC, Low Temperature Carbonitride, LTCN). This term is used to describe a large family of processes in which nitrogen and carbon are absorbed into the surface layer of a variety of carbon and steel alloys. The process uses temperatures below the transformation temperatures of the alloy. Nitrocarburizing produces wear-resistant surfaces and increased case strength. When nitrocarburizing is used, cases are very thin, so finishing must be avoided.

img-36Advantages of Nitempering
INBO-Illustrationn this era of rapidly-advancing metal technology, the demand for better performance characteristics has forced the metallurgical profession to develop improved metal processing techniques to keep pace with other segments of the metal-working industry. The needs of modern engineering for materials to facilitate reduced mass design and increased operating temperatures have frequently been answered by the development of new and improved case-hardening techniques for ferrous alloys. DIAMOND HEAT TREAT, INC. has perfected a modern new case-hardening method, the NITEMPER process. It offers many distinct advantages compared to similar case-hardening techniques already employed, particularly with regard to the quality of the resultant case and the simplicity of the processing procedure.

NITEMPER processing is a low-temperature, controlled atmosphere heat treatment applied to steel and cast iron to obtain a tough case that greatly enhances wear and endurance properties. The process is carried out at a temperature below the transformation range for steel in an atmosphere consisting of partly dissociated ammonia and endothermic gas. A combination of properties, which are highly desirable for many engineering applications, are produced.
These include:

  1. 1.A high surface hardness which is retained even after heating to as high as 1300 degrees F.
  2. 2.Increased wear resistance — particularly for metal-to-metal wear.
  3. 3.Reduced tendency to seize and gall.
  4. 4.High resistance to fatigue.
  5. 5.A tough compact case with a reduced spalling tendency.
  6. 6.Minimum distortion resulting in low finishing costs

The NITEMPER process should be considered for any applications involving wear or fatigue.
NITEMPER is performed by heating ferrous alloys of suitable composition in an atmosphere containing both a carbon and a nitrogen potential. The time required at the processing temperature depends upon the steel being treated and the depth of case desired. The principal elements contained in solid solution in steel, which assist in the formation of useful nitrides forming a hard case, are aluminum, chromium, molybdenum, vanadium, and tungsten. The hardening reaction occurs when atomic nitrogen from the dissociated ammonia diffuses into the steel and reacts with the alloying elements to form precipitates of hard alloy nitrides. The characteristic “white layer”, developed on the surface of parts subjected to a nitriding operation, appears following formation of the nitrides. The nitrogen atoms remaining in solid solution serve to increase fatigue strength by blocking the development of potential fatigue cracks. The carbon derived from the carbonaceous atmosphere is relatively insoluble in steel at the processing temperature. However, the carbon potential of the processing atmosphere has a definite effect upon the nucleating characteristics at the surface of the ferrous alloy. It is considered to be quite tough and compact compared to the white layer developed by alternative processes

Processing Variables
The processing atmosphere, composed of ammonia gas and endothermic gas, is mixed and circulated within the furnace. The raw ammonia dissociates at the hot steel surfaces into atomic nitrogen and hydrogen according to the first stage of the following reaction:

   2NH                —- 2N        +  6H            —- N        + 3H

                      3                                    2                   2

                       (g)           (g)             (g)             (g)

The atomic nitrogen, which is not readily dissolved by the ferrous alloy, and the atomic hydrogen pass rapidly through the second transformation of the above reaction. The resulting molecular forms are stable and inert, and must be exhausted from the furnace atmosphere. Dissociation rates of the raw ammonia, typically 50 to 60 percent, are influenced by the gas flow rates within the furnace. These dissociation rates should not be compared to those for other Nitriding processes without considering that the processing atmosphere is diluted with an equal quantity of endothermic gas.
The carbon is provided according to the following carburizing reaction, as a direct result of the combustion of natural gas:

  2CO                —- C     +     CO

                                                   2

                     —-   (g)              (g)

The process temperature used brings about the maximum diffusion rate of nitrogen while enhancing the carbide formation at the surface. A lower processing temperature may be used if the core hardness is a critical factor.

Preliminary Treatment
NITEMPER processed parts do not warp nor distort to any appreciable extent, provided that the internal stresses resulting from machining and heat treating are removed before processing. Hardening, then reheating at 1100 degrees to 1300 degrees to obtain the desired core hardness promotes optimum response to processing and removes stresses produced by rough machining. Final machining or grinding is usually performed just prior to
NITEMPER processing

The surface to be processed should be a machined surface, clean and free from contamination. Any scale or decarburization due to earlier forming or heat treating operations should be removed; otherwise the NITEMPER case will be very brittle and will tend to spall. Stainless steels and other steels with high chromium contents rapidly form a chrome oxide compound layer on the surface, which prohibits the diffusion of nitrogen into the case.

Metallurgical Properties
As previously pointed out, the NITEMPER processing atmosphere provides both carbon and nitrogen to the surface of the ferrous alloy. Nitrogen is more soluble than carbon at these temperatures and diffuses into the work while the carbon forms iron and alloy carbide particles at or near the surface.
The NITEMPER processed case consists of the characteristic “white layer” common to all nitrided parts and the underlying diffused layer. The properties of the white layer, typically .0001″ to .0015″ thick, are critical for most applications. It is considered to be quite tough and compact compared to the white layer developed by alternative processes. The reason for this is believed to be closely related to the rich carbon potential of the processing atmosphere and its effect upon the nucleating characteristics at the surface of the ferrous alloy. The nitrogen within the white layer precipitates out of solid solution as either alloy nitrides or carbon-bearing epsilon iron nitrides.

The nitrogen which diffuses through the white layer into the diffusion layer beneath is either precipitated as alloy nitrides or retained in solid solution. The formation of the precipitated nitrides is responsible for the hardening mechanism in the diffusion layer as it is in the white layer. However, the degree of hardening is reduced because of the diminishing concentration of diffused nitrogen.
Relative values of the white layer depth with respect to diffusion layer depth, as well as the absolute values of each depend upon:

  1. 1.Processing temperature
  2. 2.Processing time
  3. 3.Content of nitride forming elements
  4. 4.Combined carbon content

All must be considered, but for a given set of parameters, the values are readily controlled and can be accurately predicted.

Like most diffusion-related processes, the diffusion depth increases proportionately with time. Temperature is also a factor. Higher percentages of carbon, nickel, and silicon dissolved in the ferrous alloy also reduce the nitrogen diffusion rate. The influence of carbon is the most drastic.

Aluminum, chromium, molybdenum, vanadium and tungsten are the alloying elements commonly found in ferrous alloys which form stable nitrides. With increasing percentages of these elements in solid solution, the precipitation of a large proportion of the nitrogen as nitrides will considerably reduce its diffusion rate.

Aluminum forms the hardest nitride, and for this reason is commonly found in steels developed for nitriding. Chromium, molybdenum, vanadium and tungsten form nitrides which are also relatively hard. The total content of these elements in the alloy will determine the maximum attainable hardness as well as the degree of reduction of the nitrogen diffusion rate. The influence of several alloying elements and the carbon content are apparent in Table 1. The resultant hardness profile and depth of white layer are indicated for a variety of steels commonly being NITEMPER processed.

4 Hour NITEMPER at 1060° F (570°C)
Steel Sample *
Hardness converted in Rc

  • Depth”
  • 1
  • 2
  • 3
  • 4
  • 5
  • 6
  • 7
  • 8
  • Surface
  • 71
  • 49
  • 57
  • 58
  • 69
  • 70
  • 65
  • 66
  • .001
  • 70
  • 44
  • 53
  • 55
  • 67
  • 68
  • 61
  • 64
  • .002
  • 70
  • 37
  • 54
  • 55
  • 66
  • 67
  • 56
  • 58
  • .003
  • 69
  • 30
  • 51
  • 50
  • 62
  • 66
  • 55
  • 50
  • .005
  • 64
  • 26
  • 51
  • 48
  • 50
  • 63
  • 55
  • 41
  • .007
  • 45
  • 49
  • 45
  • 63
  • .010
  • 36
  • 24
  • 47
  • 35
  • 30
  • .015
  • 23
  • 40
  • 30
  • .020
  • 22
  • 30
  • Core
  • 30
  • 20
  • 24
  • 25
  • 45
  • 60
  • 55
  • 27
  • White Layer”
  • .0001
  • .0009
  • .0007
  • .0006
  • .0004
  • .0002
  • .0006
  • 1
  • Nitralloy N
  • .25C
  • 1.15CR 1.00AI .55Mn 3.5Ni .25Mo
  • 2
  • 1015
  • .15C
  • .50Mn
  • 3
  • 8620
  • .20C
  • .50Cr .20Mo .80Mn .60Ni
  • 4
  • 4140
  • .40C
  • 1.00Cr .20Mo .90Mn
  • 5
  • H13
  • .35C
  • 5.00Cr 1.00V 1.50Mo
  • 6
  • M2
  • .80C
  • 5.00Mo 6.00W 4.00Cr 2.00V
  • 7
  • D2
  • .1.55C
  • .30Mn 12.00Cr .90V .80Mo
  • 8
  • 410 Stainless
  • .10C
  • .30Mn 12.50Cr
  • All samples were hardened, tempered and then NITEMPER processed for four (4) hours.
  • The surface hardness was measured by determining the Knoop hardness number on a buffed processed surface which was machined to a fine finish before processing.
  • All hardness were converted to Rc after being measured with a Tukon Microhardness Tester, using 500 gram load.

Applications And Related Advantages
The NITEMPER process can advantageously be substituted for such case hardening techniques as salt bath nitriding, gas nitriding, carbo-nitriding, carburizing, and induction hardening. Typical components which have been successfully NITEMPER processed include camshafts, connecting rods, corrugated rolls, crankshafts, die casting dies, gears, retaining rings, shafts, splines, and wear plates. Steels from almost every classification have been NITEMPER processed. Field performance tests, wear tests, and fatigue tests often involving direct comparisons between similar components processed by other case hardening techniques, have been reported.

The NITEMPER process offers the following advantages:
1.Tough Ductile Case — The carbide formation promotes a tough ductile white layer which has a reduced tendency to spall.
2.Less Processing — The ductile white layer does not ordinarily require grinding.
3.Short Cycle —A satisfactory case is produced with minimum furnace turn-around time.
4.Applications — Reduced spalling tendencies allow it to be used on a greater selection of ferrous alloys.

The advantage most commonly realized when the NITEMPER process has been substituted for carburizing, carbo-nitriding or induction hardening has been less distortion and growth. This results in minimal finishing cost. Often a material can be up-graded by the NITEMPER process or an improved product can be made at an equivalent or even a reduced cost.

NITEMPER is now a highly accepted process used to improve wear and fatigue properties of ferrous alloy components. It is a contemporary economical process compatible with and designed for the requirements of present day industry.