Sinter Hardening

Powder metallurgy alloys that can transform to the hard martensite phase upon cooling from the sinter temperature have been commercialized. This technique, termed sinter hardening, is rapidly gaining popularity in applications requiring high strength and hardness immediately after sintering. This manufacturing route has proven to be cost effective for a number of parts because it eliminates the need for a postsinter heat treatment.

Prealloyed powders have been proven useful for this purpose. The ability of the alloy to promote the formation of martensite and/or bainite during cooling from the sinter temperature is the most important factor to optimize the microstructure and therefore the properties of the sintered parts.

Manganese is added to all commercial steels in the range of 0.25 to 1.0% to increase strength and hardenability of plain carbon steels. Chromium is also a popular hardening element, but it is seldom used in prealloyed powders because of the difficulty in reducing the chrome oxide that can form at the surface of the particle and act as a barrier to interparticle diffusion. Molybdenum and nickel are most commonly used in low-alloy P/M steels because of their easily reducible oxides. These elements are very efficient in promoting increased strength and toughness. Sinter hardening requires controlled cooling after sintering in the austenite range (1120-1290 °C, or 2050-2350 °F). The factors that determine the ability of an alloy to sinter harden are cooling rate between 600 and 150 °C (1110 and 300 °F), which encompasses the martensite start (Ms) temperature of most P/M alloy steels, the alloy content, and the blend composition. The most commonly used alloy is type 4600 with additions of copper and carbon.

An optimal microstructure for a sinter-hardened material would show more than 90% martensite at the surface of the part with no less than 70% martensite in the core. This structure would provide the surface hardness and toughness of a quenched microstructure without the high stress concentration resulting from oil quenching.

Therefore, it is important to know the cooling rate and alloy content that provide the required hardenability for a part of a given mass and density. In a standard belt-type sinter furnace, an addition of 2% Cu and 0.9% C to the 4600 alloy may be necessary to provide sufficient hardenability for a given part density. With the new furnaces that are available with enhanced cooling capability, a lower alloy content or mix composition can be used to achieve the same level of hardenability.

A simple laboratory method has been developed to determine the effect of cooling rate, density, and alloy content on the hardness and microstructure of a sintered material. Cylindrical slugs of varying mass and density are sintered with thermocouples embedded at the surface and center of the slug. Another thermocouple is fixed on the belt to measure the ambient cooling rate of the furnace set at a known temperature and belt speed. Sectioning the slugs to characterize the microstructure and hardness profile at the location of the thermocouple provides the necessary information to construct a diagram as shown in Fig. 15, which shows the results for a type 4600 alloy. These microstructure-hardness-cooling rate diagrams can be constructed for various alloy blend compositions and density levels to find the most cost-effective material for a given application.

Fig. 15 Cooling rate-hardness-microstructure diagram to determine hardenability requirements. Source: Ref 17

There are several advantages to sinter hardening besides cost effectiveness:

• Microstructures can be better tailored to provide optimal strength and hardness.

• The porosity remains free of oil, which is advantageous to parts operating in controlled environments.

• Distortion in complex-shaped parts is reduced compared to oil quenching.

This reduced distortion translates into improved dimensional stability and consequently higher yield and quality of production lots.

The major drawback of sinter hardening is that the alloys used require a high carbon content to achieve the necessary hardenability. As has been shown previously, high carbon levels reduce the mechanical properties of P/M steels after hardening. Table 2 shows a comparison of properties of a type 4600 alloy with 0.5% C that was quench and tempered with a 4600 alloy (with a 0.8% C level) that was sinter hardened. As with quenched alloys, sinter-hardened alloys require post tempering treatment to attain the optimal combination of hardness and toughness.

Table 2 Comparison of properties of quenched-and-tempered type 4600 alloy (0.5% C alloy) with a sintered hardened 4600 alloy (0.8% C)

Material code

Density,

Ultimate tensile

Apparent

g/cm3

strength

hardness, HRC

MPa ksi

FL-4605-120HT

6.95

895 130

34

0 0

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