Heat Treater Avoiding cold temperature problems

One reason the Titanic sank!

Avoiding cold temperature problems

When the physical properties of a steel are reported, the value shown for toughness (which may be defined as resistance to failure under conditions of shock loading) should always make reference to the temperature of the test because lower temperatures lead to lower strengths.
The movie "Titanic" tells the story of a mechanical failure leading to a disaster. There were many contributing factors but one which gets little mention was the poor quality of the steel in the hull.
Samples recovered from the wreck were tested for impact strength . The tests were conducted at a few degrees above freezing - the same conditions that were encountered the night of the sinking. The impact resistance was found to be very low, so that instead of yielding and buckling under impact, the steel failed in a brittle manner causing severe damage and catastrophic results.
The low impact strength was due to the high level of impurities - levels which would cause rejection under today's standards. The important point here was the temperature of the test.
For any steel, the impact strength (within the range +300F to -200F) will be lower at lower temperatures, however the rate of loss of strength is not uniform, and many steels will experience a rapid loss of strength over a relatively small temperature drop. This is known as the ductile - brittle transformation zone. The loss of strength can be as much as 30-40%
This will often occur in the range of temperatures which we experience between summer and winter and it is important to select a steel and a manufacturing process which will give the desired properties at the lowest temperature to which the steel will be taken.
The factors which influence toughness are:

The alloy content and the level of impurities in the steel;
The prior mechanical and thermal treatment which affect grain size and microstructure;
Surface conditions.

A surface which has machined notches or rust pits which act as stress raisers is at a definite disadvantage.
The first step in reducing the risk of low temperature brittle failure is in the selection of an appropriate steel alloy. Use a steel which has sufficient alloy content to allow for full hardening during heat treatment. Look for impact strength values at the relevant temperature.
Once the steel is selected it is important that it is given a correct heat treatment which will produce a fully martensitic (i.e. fully hardened) structure. The important stages are as follows:

Raise the steel to the appropriate temperature and hold for sufficient time to allow full dissolution and mixing of all constituents.
Maintain an appropriate furnace atmosphere
Assure rapid cooling by use of adequate spacing of parts using a quenching oil of a suitable speed and a rapid agitation.
Temper to produce the correct hardness

At any level of hardness this will give the greatest low temperature toughness.
Following these guidelines will ensure that the ductile/brittle transformation is depressed to its lowest point and the combination of strength and toughness is achieved.