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What’s up guys? This is Michael with Michael Talks Metal back for some more metal fun! You ready? Well I am! Let’s jump in!
So let’s start with just a bit about expansion
Metals generally expand with increasing temperature and it is expressed as the “Coefficient of Thermal Expansion” (CTE for short) and in english units would be inches per inch per degree F x 10 to the minus 6 . Another way to say this is the CTE is how many millionths of an inch a one inch dimension changes for every degree of temperature change.
Things are more complicated in that the CTE also varies with temperature.
To keep the math easier and for us to make better comparisons, the MEAN ( the average) CTE over a range of temperatures can be used. Tables of data show the mean CTE for the temperature range of interest …… With this data, we can compare two materials or we can make the calculation of how much a dimension will change with a change in temperature for an alloy.
An ordinary steel or aluminum alloy has a mean CTE of about 12 ( x 10 to the minus 6) or point 000012 inch per inch per degree in a temperature from 70 to 300 degrees F. So if we have a one inch part at a 70 degree F temperature and heat it to 170 degrees F it would measure one point 0012 inches at 170 degrees. It grew by just over a thousandth of an inch.
But what do we do if we need to minimize any changes with varying temperature or if we want to match expansion to a different material?
Invar 36, UNS K93600 is essentially 36 % Nickel and the balance iron.
CTE is 1.17 (x 10 to the minus 6)…. one tenth that of our carbon steel or aluminum example.
This alloy is used where stability with temperature change is important such as electronics and aircraft controls and laser and optical systems
We could also join it to a metal with a higher CTE to generate motion like an old style mechanical thermostat switch.
A free cutting version UNS K93601 has point 20 % selenium added to aid chip formation in machining of parts.
ASTM B573 is a common specification
Kovar, UNS K94610is an alloy of 29% Nickel, 17% Cobalt and the balance iron
In this alloy, its CTE characteristics are both consistent over a wide temperature range and compatible with the CTE of many glass and ceramic compositions making it ideal for forming glass to metal seals. Too much difference would lead to breaking the seal or stresses that could fracture brittle glass or ceramic components
Typical specification ASTM F-15
There are other alloys used for specific applications but these two alloys represent the principals involved.
As always specs for material and for parts often carry additional requirements.
Check the specs!
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