Cast Irons and Related Processes

Author: George Idriceanu*

A principal property of an automotive stamping die is the wear resistance which is directly related to the surface hardness and surface finish. The formation of a pearlitic structure will retain good strength and wear resistance after subsequent treatments.

Certain elements, such as carbon and silicon, accelerate the decomposition of pearlite and massive carbide at the annealing temperatures. An important aspect related with the success of flame hardening is the combined carbon content, which should be in the range of 0.5 to 0.7%. Flame hardening is not recommended for irons that contain more than 0.80% combined carbon because of the susceptibility of cracking (ref. 1).

Since silicon promotes the graphite formation and decreases the combined carbon on the other hand, it is advisable that it should not exceed 2% in any iron submitted to flame hardening.

The pearlite-promoting elements (manganese, chromium, phosphorus, nickel, copper...) delay pearlite decomposition. The percentage increases in the time required to decompose pearlite that are affected by 0.1 % addition of these elements are:

**Ref. 1

As we can see, chromium is the most effective pearlite promoter (6-7 times more effective than copper or nickel). It is also a strong carbide former and stabilizer in the flame hardening time. The flame-hardened gray iron typically has a lower hardness than the metal immediately below the surface due to the retention of austenite at surface (ref. 1)

Mo and Mn are the recognized elements for increasing the hardenability of gray iron (the Jominy

curve is shifted to larger distances from the quenched end). It is advisable that Mn be held at 0.8 to 1.0% to increase carbon solubility in austenite.

Ni increases the hardenability, impact load resistance, elasticity modulus and decreases the temperature of the transformation points.

Cu decreases the impact resistance and it has a much lower contribution in improving the hardenability and elasticity modulus.

Both, Ni and Cu, are weaker hardeners and do not affect the hardenability nearly as much. These elements tend to segregate during solidification (ref. 2) and to form a "sticky" surface with nonuniform hardness distribution. Also, they do not increase the hardness of the first 0.020-0.040" below the surface which are the most important in a sliding wear application (fig. 1, ref. 3) and in concentrations over 1.0% they tend to retard nitrogen diffusion.

In cast irons, aluminum is a graphitizer and it can be found as an "impurity" originating from the

ferro-silicon added to the melt. Like Cr, because of its high affinity for nitrogen, aluminum is responsible for the increase in surface hardness after nitriding at values over 1100 HV (ref. 1, 4).

The hardenability is particularly of importance for relatively small heat treated parts where the bulk

properties are important and adequate cooling rates may be obtained. For large automotive dies, weighing several thousand pounds, where the bulk properties are not achievable because of their sizes, surface treatments, such as flame hardening, ionitriding or chrome plating, are used to improve the sliding wear resistance and hardness of the surface.

The consideration of residual stress is divided into two major orders:

• the First-Order Stress is defined as macro or long range residual stress acting over large regions and phases in the polyphase materials. It represents over 90% of the total stress which can be found in stressed bodies.

• the Second-Order Stress can be defined as micro, short-range or texture stress across one grain or part of one grain of the material (thermo-mechanical induced lattice defects, precipitates,.. .). It represents less than 10% of the total stress and it is usually ignored when designing mechanical parts (ref. 5).

By heating the iron (at a rate not higher than 150 to 200°F per hour), the stress (from both classes) is

relieved by rapid creep. Very large shapes and complex machined parts should be cooled uniform into the furnace even under 200°F, so that stress is not reintroduced.

Stress relieving by vibrations has been promoted in the market, as a method for providing stress

relief to iron castings. This procedure has not been demonstrated to be successful in a valid test (ref. 6), nor to remove any of the first class stress which involves long range relaxations or structural modifications. The influence over the second class is still controversial.

Flame hardened castings should be stress relieved to minimize distortion or cracking and for

increasing the toughness of the hardened layer. At 300 to 400° F after 6 to 8 hours, 15-25% of the stresses can be relieved while reducing the surface hardness with 2 to 6 HRC depending on the alloying elements content (fig. 2, ref. 1). At higher temperatures (800 to 11 OO°F) up to 90% of the stress can be relieved while the surface hardness may decrease with 6 to 12 HRC (fig. 3, ref. 1).

Cast irons can be chrome plated if the surface is capable of conducting the required current and is free of large voids, pits, gross inclusions and massive segregation. Also, the surface must be free from macro-stress or surface stress induced during hardening, grinding, stoning and polishing, which may result in microcracks after plating.

Large and intricated castings with a hardness of over 40 HRC should be stress relieved before plating at 300 to 450°F. To ameliorate the effects of hydrogen embrittlement these parts should be baked at 350-450°F for 3-4 hours, immediately after plating (see ref. 7 and Federal Specification QQ-C-320, Amendment 1).

As an alternative to chrome plating, IONITRIDING®, a clean and pollution free process, is often used effectively because of its specific advantages and cost effectiveness:

• increases impact load and sliding wear resistance at once with the uniformity of the hardness distribution over the entire surface of the processed die

• improves fatigue resistance against cracks initiation into the surface due to compressed stresses ~ developed

• deeper case depths: 0.008 to 0.012" diffusion zone plus 0.0004 to 0.001" extra hard epsilon layer (indicated for sliding wear applications)

• advanced stress relieving (75 to 85% and vacuum outgassing after casting, grinding, welding repairs and flame hardening, in the same process

• because the layer is diffusion based, it will not flake or peel and it has low friction (typically 0.10 against steel), good antigauling and antiscoring properties due to the ceramic nature of the nitrides

• after service or if emergency repairs are needed, the damaged spots can be successfully welded "in house" many times (the whole cast is stress relieved) and without the need to immediately reprocess the whole die.

The decision in choosing a process or a combination of processes which should be used to meet the requirements of a specific application is an analysis of the given existing circumstances, examination of use requirements for the parts, and any outside factors (time availability, costs, production volumes, equipment tolerances,.. .).

REFERENCES

1. B. Kovacs, Heat Treating of Gray Irons, ASM Handbook, Vol. 4, 1991, p. 671-692.

2. E. Dorazil, B. Barta, E. Munsterova, L. Stransky, A. Huvar, High Strength Bainitic Ductile Iron, Int. Cast. Met. J., June 1982, p. 52-62

3. Y.H. Lee and R.C. Voight, The Hardenability of Ductile Irons, Trans. AFS, Vol. 97, 1989, p. 915-938

4. U.H. Gommann, Gas Nitriding: Choosing Metals For Success, Heat Treating, March 1991, p. 22-23 5. A.K. Sinha, Defects and Distortion in Heat-Treated Parts, ASM Handbook, Vol. 4, 1991, p. 603 6. C.F. Walton, Introduction to Heat Treating of Cast Irons, ASM Handbook, Vol. 4, 1991, p. 669 7. ASM Committee, Hard Chromium Plating, Metals Handbook, Vol. 2,1964, p. 462-474

* George Idriceanu, Director of Research and Development, and Process Engineering, Sun Steel Treating Inc.