Process for treating steel alloys

Abstract

The process includes heating of steel alloy parts in the presence of hydrogen prior to introduction of, for example, carburizing gas, preferably acetylene, in the presence of a diluent carrier gas. The process uses a continuous cycle involving only one carburizing (boost) step and one diffusion step, and high velocity gas quenching. The process is advantageously carried out in a unique, versatile vacuum furnace (also having high internal pressure capability) that provides very high velocity, continuous flow gas quenching, and a furnace design, including a long, low profile work zone configuration and quench gas recirculation equipment and flow pattern that facilitates high velocity gas flow. The entire process is accomplished in a single self-contained chamber of the furnace.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to low pressure carburization and other heat treating processes applied to metal alloy parts and more particularly steel parts and to high temperature capable furnaces having the capability of providing in the same furnace chamber alternatively, low pressure (vacuum) and high pressure (gas quench) environments for such processes. [0003] 2. Background Art [0004] Vacuum (low pressure) heat treating (carburization) of steel or high alloy-content steels has been accomplished over past decades using various heat treating processes. Some alloys are particularly difficult to treat and require post treatment, for example, quenching to finish the treatment. Some metals are more difficult to treat (for example alloys such as AISI grade 4140, 4340, 8620, and 9310). Work pieces containing such alloys are currently heat treated and then moved to an oil or salt bath quench. That is, the work pieces a...

AI Technical Summary

Benefits of technology

[0006] Applicants have found that a carburizing process including heating of steel parts in the presence of hydrogen prior to introduction of carburizing / diluent gas, can provide substantial improvement in carburizing in accordance with the present invention. The process uses a continuous cycle involving only one carburizing (boost) step and one diffusion step, and carburizing gas, preferably acetylene in the presence of a diluent carrier gas. The carburizing is desirably carried out in a furnace having high velocity quenching capability. The process according to the instant invention uses hydrogen as a pretreatment gas with significant soak time under heat, then, after the pretreatment, carburizing, followed by a high pressure, high velocity gas quench. The process provides a method that avoids the need for: (a) a highly programmed cycle; and (b) a complex sequential boost / diffuse process. The process also substantially avoids the requirement for sand blasting the steel parts prior to carburizing. The process is advantageously carried out in a unique, versatile furnace that provides a novel, high velocity, continuous flow gas quenching capability, and a furnace design, including an effective work zone configuration that contributes to more effective carburization. The entire process is advantageously accomplished in a single self-contained chamber of the unique furnace. The advantages of a high velocity gas quench are substantial. For example, with the gas quench there is far less work piece distortion and no oil cleanup following heat treatment. Also, the cost of having a separate chambers and equipment for moving workloads from one chamber to another are completely avoided.

Problems solved by technology

Some alloys are particularly difficult to treat and require post treatment, for example, quenching to finish the treatment.

The pieces thus moved and quenched have problems with distortion.

Also, cleaning the parts after they have been submerged in oil or salt is a costly challenge.

The mechanism for moving the work pieces at temperature undesirably adds significant cost, time and maintenance issues to the process.

Although, gas quenching avoids much of the finish product cleanup issues, it does not avoid the mechanical movement of the workload from one chamber to another.

It also is not without challenges in how it affects finished product quality.

With such hydrocarbon gases, however, soot produced in the carburizing chamber interferes with desired consistency of carburizing quality and adds significant cost to parts cleanup and furnace maintenance.

Achieving a uniform carburized “case”, a hardened, uniform surface layer, has been difficult and costly.

Uniformity has been a major challenge.

Atmosphere carburization suffers from the added problem of surface oxidation during heat treatment.

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