Super Alloys

Relaxation occurs when metals are subjected to increased high temperatures and stresses, resulting in permanent deformation such as a load loss at a constant deformation or creep, an increased deformation at a constant load. Coil springs are designed to give a controlled deflection at certain load, and at temperatures above 500°C, low alloyed metals and stainless steel coil springs have low relaxation, jeopardising their performance. This is why at European Springs we use super alloys to ensure a high performance from our range of springs is given at all times even when encountering such high temperatures.

Super alloys are formed with a model of austenitic face-centred cubic crystal structures, with a usual base element of nickel, nickel-iron or cobalt. Super alloys develop their high temperature strength through solid solution strengthening, with oxidation and corrosion resistance formed by a thermal barrier coating, usually complied of aluminium or chromium elements. This coating establishes its’ protective covering when the metal is exposed to oxygen and completely encases the metal, saving it from temperature and stress damage.

Applications for the use of super alloys are dominantly used in aerospace components, turbines and automobile production.

Super alloys tested

Spring Engineers at European Springs carried out experiments on three different super alloy wires and one stainless steel. The main chemical analysis and the mechanical strengths results are presented in the following table:

Note * Grade 17/7PH

Relaxation test method

A relaxation test was also carried out in conjunction with a number of coils manufactured to standard production methods. The springs were then compressed with a static load to a specified stress level with help of a mechanical joint. The compressed springs with joints were exposed to specified temperatures for different times. After cooling, the load for each spring was tested again and the loss of load was noted. The relaxation figure presented was defined as follows:

(F_b-F_a) / F_b ∙ 100 = Relaxation (%)

F_b = Load at compression before heating

F_a = Load at compression after heating

Following times and temperatures were used with different stress levels at each:

Results

The stainless 17/7PH steel failed as expected after 50 hours at 550°C and stress level 150 MPa. The relaxation received was close to 100 %. No more tests were made with this grade.

Conclusions

The results show that stainless steel cannot be used at increased high temperatures. It also shows the big difference between different super alloy grades. From tested super alloy grades the C grade had the lowest relaxation and performed best at all temperatures. It can also be seen that depending on alloy, temperature and stress the relaxation has to be considered when designing coil springs to be used in applications where high temperatures can be expected.

Read more:

Super Alloys – (external link)