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Higher tensile strength for alloy steel 4340 screwHelpful Member!(3) 

gioasginc (Aerospace) (OP)
24 Jun 10 10:46
May be someone can help me in this:
I have a screw :#10-32 (diameter .19", length 1")hex head, full thread, alloy steel (4340), that according to NAS1801 it has 160 KSI tensile strenght.
Can I heat treating it, to get 300 KSI tensile strenght, if the case what would be the treatments, temperatures and times.
thanks in advance  
CastMetal (Mechanical)
24 Jun 10 12:48
According to these specs 260ksi is on the upper end of heat treatment strength for 4340.
http://www.suppliersonline.com/propertypages/4340.asp#Spec
Helpful Member!  gioasginc (Aerospace) (OP)
24 Jun 10 13:24
The specs for this particular screw says: heat treat: 160 to 180 KSI tensile strength.
I just want to know if this material can get better tensile strength and what are the steps in heat treating to get that.
thanks  
Helpful Member!  CoryPad (Materials)
24 Jun 10 13:32
You can get ultimate tensile strength between 250 ksi and 300 ksi using this process:

austenitize at 800 °C
oil quench
temper at 225 °C

You must be careful with material in this condition.  Low elongation, low fracture toughness, low impact resistance, and low resistance to hydrogen assisted cracking and stress corrosion cracking are all strong probabilities.
gioasginc (Aerospace) (OP)
29 Jun 10 8:57
Thanks
The screw will be use inside of the cockpit(airplane), for some attaching device.
can you be more specific about the process
austenitize at 800c but what time?
oil quench  what Temperature of the oil?
temper how long?
 
Mater if the screw already has a treating process,
(as specs indicated)
do I need to do a previo process?

Thanks in advance!!
TVP (Materials)
29 Jun 10 10:54
The hold time at 800 C is 15 minutes for each 25 mm of section/thickness, or a minimum of 15 minutes.  Quench in warm oil at 25-60 C.  Since the part is already threaded you will want to use as high a temperature as possible.  Temper at least 2 hours.

As CoryPad mentioned, this will result in a very brittle microstructure, with low fracture toughness.  And since you will be eliminating the residual compressive stresses in the shank-to-head transition that were introduced by fillet rolling, this fastener will be very susceptible to SCC and fatigue failures in this area.  Do you intend to have this part electroplated after heat treating?  If the answer is yes, you will now have a very brittle fastener that has been exposed to hydrogen, and therefore hydrogen embrittlement and delayed fracture are a signficant concern.

Based on all of this information, you would be well advised to find a proper aerospace-quality fastener rather than re-work an existing part.
Helpful Member!  Wrenchbender (Mechanical)
29 Jun 10 14:37
Possible but not advisable.  Airplanes vibrate and the temperatures can be very cold at altitude.  As mentioned previously at 300 ksi the frax toughness, fatigue properties, etc. would be pitifully low.  Cold temperatures will make these even worse.  Also the thread is spec'ed at class 3(close tolerance).  They will have to be cleaned up after any re-heat treatment.  Best approach would be to use another type of material for the screws or more of them.

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