Microstructures
by George Langford, Sc.D., Massachusetts Institute of Technology, Cambridge, MA, 1966
Copyright©2005 by George Langford
Low Alloy Steels - Lesson 2 - Sixth specimen
0.6% carbon steel at 500X etched
 This 0.6% carbon steel cracked after it was brazed (soldered with a high melting copper alloy) to a brass bellows.  Evidently, the martensite shown at left at 500X formed upon cooling to room temperature.  Note the manganese sulfide stringers in this free machining steel.

The microstructure consists of ferrite, pearlite and martensite.  The martensite is the brownish unresoved microconstituent.  The pearlite is black and also is unresolved.


Remedies ?       Pause; then take a look at the answer.



























Answer:  There are several choices, any or all of which would eliminate the problem if done correctly:
(a) Change the cooling rate by furnace brazing or by by preheating the entire fabrication;
(b) Post heat after brazing to temper the martensite which forms on cooling from the brazing temperature;
(c) Lower the brazing temperature below the eutectoid temperature of the steel by switching to silver solder; or
(d) Use a lower carbon, less hardenable steel.

In this case, the 0.6% carbon steel had been used in error; it should have been a lower carbon steel as originally specified.
 Now go to Specimen 7.