Cryogenics, or deep freezing is done to make sure there is no retained Austenite during quenching. When steel is at the hardening temperature, there is a solid solution of Carbon and Iron, known as Austenite. The amount of Martensite formed at quenching is a function of the lowest temperature encountered. At any given temperature of quenching there is a certain amount of Martensite and the balance is untransformed Austenite. This untransformed austenite is very brittle and can cause loss of strength or hardness, dimensional instability, or cracking.

Quenches are usually done to room temperature. Most medium carbon steels and low alloy steels undergo transformation to 100 % Martensite at room temperature. However, high carbon and high alloy steels have retained Austenite at room temperature. To eliminate retained Austenite, the temperature has to be lowered.

In Cryogenic treatment the material is subject to deep freeze temperatures of as low as -185°C (-301°F), but usually -75°C (-103°F) is sufficient. The Austenite is unstable at this temperature, and the whole structures becomes Martensite. This is the reason to use Cryogenic treatment.

Cryogenic treating is usually performed by quenching a part from room temperature, into a bath of liquid nitrogen. In steels, the temperature where the completion of martensite formation from austenite can be substantially low. If this temperature is never reached, the microstructure will contain retained austenite. After tempering, the microstructure will contain tempered martensite and retained austenite. A cryogenic quench will finish the transformation. At this point, the microstructure will contain tempered martensite and un-tempered martensite. You must re-temper the part after this operation to relieve the stresses.

 

There are several aspects of the De Lorean car's design and construction that are fairly advanced.
The cryogenically manufactured torsion bars that counterbalance the doors were developed and manufactured by Grumman Aerospace. Their function is taken for granted by DeLorean owners but the design and function is very elegant.
Previous gullwing cars struggled with counterbalancing the gullwing doors. The Mercedes sports coupe tackled the problem by making the doors very light. For structural reasons, the door sills on the Mercedes therefore had to be very substantial and the sills are quite high. The Bricklin approached the problem by using pneumatically operated struts that would raise the doors using compressed air stored in its rear bumper. The doors would get pumped up at the press of a button. This approach was often slow and cumbersome. The DeLorean torsion bar counterbalances the door by storing torsional energy (in a way similar to how a spring stores energy when compressed). The cryogenic manufacturing process was necessary in order to strengthen the torsion bar for repeated operation without the bar fatiguing or breaking. In a process that I do not understand well, the crystalline structure of the material was modified in the process so that the "grain" in the material would run through the bar in an optimal way. If you examine a DeLorean torion bar you will see that it is twisted along its axis several times over its short distance. I have never heard of a torsion bar breaking under normal use, although I have heard of people over doing the pre-tension in the bar and twisting the roofs of their cars.

Making the gullwings work
John Lamm relates the story of the De Lorean's gullwing doors in his book "De Lorean Stainless Steel Illusion': Grumman aircraft was looking for an anwer to a structural problem when Collins also mentioned that he hadn't yet found a truly satisfactory means of getting the gullwing doors up smoothly and with minimal effort. "The first one that Mike built," Collins explains, "had four little 1/8 inch diameter rods that went forward into a block of steel, sort of welded in there, and then four that went back the other way. It was always making noise and one of the bars would slip and then it wouldn't work. So, I said, "I need a bar that is about 32 inches long that I can mount back in the roll bar and then forward that will open this heavy door." Their development guy, Art August, said, "Let me give that to our Dr. Phoo-phoos, and we see what they can do". What Grumman had was a cryogenically stressed torsion bar. They put the bar, which has been stretched and straightened, in liquid nitrogen, at minus 320 degrees Fahrenheit, and twist it through 10 revolutions. The grain structure of the bar is rearranged and during the process it gets neither smaller in diameter nor shorter. Said to be good for 50,000 opening and closing cycles without losing strength, Grumman's 'crysotwist' bar became the ideal torsion bar for the De Lorean doors.

Pictures, top: cryogenically treated brake disk; right: De Lorean

 

1) Explain the conversion from Fahrenheit to Centigrades.

2) Show the relation between Kelvin and Centigrades.

3) Make a sketch of the iron-carbon phase diagram and show the solutions of iron with 0.4%, 0.8% and 1.2% carbon at various temperatures.

4) Tell us something of the De Lorean car. Who was its originator? Where was the De Lorean factory?

5) Why was the De Lorean built with gullwing doors? Are there any other cars with gullwing doors? Make a list of pros and cons for gullwing doors.

6) Name more examples where torsion bars are used.

7) What sort of steel is used for torsion bars?

8) What do know about the Grumman Aircraft Company? Who founded the company and when? Name some of its planes. Does Grumman still exist?