The Navy C Ring Test – A Practical Tool for the Heat Treater
Heat treaters are forever curious about how their furnaces are performing; in particular the uniformity of properties that will be achieved throughout the load. We often look to sophisticated tools for answers, but there is a simple, yet highly effective, method for quantifying our furnace’s performance – the Navy C-ring test. Let’s learn more.
Originally designed to study dimensional changes that occur in the heat treatment of hardened and/or case-hardened components, the power of the Navy C-ring test is that it can be adapted and used while running production loads to determine both the overall performance capability (i.e., condition) of the furnace and the heat-treatment process being conducted in it. It can also be used to compare in-house heat treatment with that of outside commercial services.
This test can be structured in such a way as to aid in the evaluation of atmosphere or vacuum furnaces. Processes such as normalizing, hardening and case hardening can be examined and the performance of oil or high-pressure-gas quenching methods studied. The test can be extended from its original purpose to reveal the following types of information (as a function of position within the workload) in both the “as-quenched” and “as-tempered” condition:
- Hardness uniformity (surface, core)
- Dimensional change (distortion)
- Quench system (oil type, agitation, temperature) effectiveness/uniformity
- Carburizing uniformity (effective and total case depth plus case variation by position)
- Microstructural uniformity (including retained austenite levels)
- Material hardenability
- Material surface stress state
- Sensitivity to cracking (as a function of various quench conditions or media)
One is not limited to a particular material grade. As such, C-rings are made from both ferrous (e.g., steel, stainless steel, tool steel) and nonferrous (e.g., aluminum, titanium) materials. For steel parts, SAE 1010, 4140, 4340, 8620 and 9310 are typical examples. It is important that the C-rings are made of the same material (and ideally from the same heat) as the production parts.
What is a Navy C-ring?
Essentially, the Navy C-ring is a short cylinder with an eccentric hole and open in one extreme (Fig. 1). The original design (specified to conform to U.S. Navy Department Specifications for Tool Steels, No. 47S5c, July 1, 1921) has a thickness of 25 mm (1 inch). Modifications of this design are common to mirror the size and thickness of the actual parts being processed. It is important to run C-rings of the same physical dimensions within a given load.
How to Conduct the Test
All parts should be measured before and after heat treatment using a coordinate measuring system (CMM) to precisely determine geometrical dimensions. This is critical for subsequent statistical analysis of the data. The Navy C-ring samples can then be positioned in a workload in either a vertical (if an optional hole is drilled in the specimen for hanging) or horizontal orientation. Typically, a minimum of nine rings is used, positioned in the corners and center of the load (like a temperature uniformity survey) along with production parts. The rings can also be positioned in individual baskets stacked to make up a load.
Testing of samples in both the as-quenched and as-tempered condition should be done for surface and core hardness, microstructure, case depth (via microhardness), retained austenite and (if desired) residual stress via X-ray diffraction (XRD) to obtain a complete data set.
Original Test Focus
Historically, the main focus of the Navy C-ring test has been to evaluate dimensional changes. In simplest terms, distortion of an engineered component can be defined as a change in its shape or volume during either manufacturing (including heat treatment) or in service.
Distortion during quenching is the result of differential volume changes due to heat extraction and/or phase transformations. These dimensional changes can dramatically influence manufacturing productivity due to necessity for post-heat-treatment machining operations. Furthermore, when distortion is severe, the potential for crack formation becomes a paramount concern.
The major factors that influence distortion are cooling rate, hardening treatment (e.g., carburizing, ferritic nitrocarburizing), material hardenability and chemical composition.
Investigating these factors has revealed that the rate of cooling from carburizing is extremely important. Very fast cooling rates (e.g., water quenching) completely outweigh the effects of major changes in composition. By contrast, while the carburizing process reduces dimensional movement in low-alloy steels, it has a lesser effect in steels of high hardenability.
Steel composition is more complex, and distinctions must be made between the effect of composition on increasing hardenability and the effect on the depression of the martensite-start temperature in fully hardenable grades. Both aspects must be fully understood to correlate dimensional movement over a wide range of compositions. To illustrate this point, the distortional behavior of boron steels is entirely different from non-boron steels of comparable hardenability.
The specification of steel chemistry to restricted-hardenability grades has been reported to reduce the variability of distortion. Even greater effect has been found by the use of steels in which the hardenability is greater than that required for through-hardening for a given section size.
The Navy C-ring has been effectively used to evaluate the final distortion produced by quenching of steel parts. This simple specimen can provide distortion information as a function of heat-treatment condition and position within the workload with respect to changes in the:
- Gap width
- Cylindrical dimensions
- Bore (if an optional hole is machined in the sample)
- Change in size on deep freeze or cryogenic treatment and single or multiple tempers (as a function of both temperature and time)
In addition, one can understand the effect of material (composition) and initial microstructure on size and shape distortion, retained austenite and residual stress.
Furthermore, the test can help evaluate the effectiveness of prior hardening processes such as (mill) annealing or normalizing prior to carburizing. Retained austenite evaluation of carburized components is another parameter of interest, especially in carburized steels that can contain varying amounts of retained austenite in the quenched-and-tempered condition – depending on the material (i.e., alloying elements) and process parameters used (e.g., carburizing and hardening temperature, carbon potential, cooling rate). Retained austenite can influence near-surface hardness and dimensional stability over time.
Heat-treat processes also create residual stress in a material, which results in dimensional variation (i.e., size and shape changes) both within a given part and from location to location within a workload.
In this day and age of uncompromising part quality, the inclusion of Navy C-ring testing on a quarterly basis will prove an invaluable aid in reducing both equipment- and process-related variability. Testing of rings for other than just dimensional change can serve as confirmation of process stability with regard to such items as temperature variation throughout the load, quench effectiveness, surface and core hardness differences as a function of position and overall material hardenability. The results of these tests can help qualify process capability, evaluate process (recipe) changes, confirm the validity of control instrumentation, direct maintenance activities and act as an invaluable quality-control tool (especially when statistical data analysis is performed). As they say, “Just do it.” You’ll be glad you did.
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