1. Strengthening Process of Aerospace Ultrahigh
Strength Steel
Oliver Mawodza - supervisor Dr. Luo Quanshun
Aeronautical Engineering (BSc)
AIM & OBJECTIVES
Investigating the strengthening of ultrahigh
strength steel using heat treatments such as
normalising and annealing as well as using
cooling methods such as quenching.
 To repeat the conventional strengthening
heat treatments and characterize the
microstructure.
 To investigate the effect of cryogenic
cooling on retained austenite elimination.
 To investigate the kinetics of tempering
treatments on selected temperatures and
times.
RESOURCES
Testing samples (of high strength steel)
EQUIPMENT
 -Hardness testing machine
 Optical and electron microscopy
 -Heat treatment furnaces
 -X-ray diffraction machine
 Tools for sample preparation (e.g. SIC
disc saw, SIC papers, diamond slurry
microns)
METHODOLOGY
 Sample sectioning by SIC disk saw
 Sample preparation procedure
 Heat treatment
 Vickers hardness HV at 30kg load
 Optical
 Low-magnifications at 2,000X AND
5,000X to characterize the martensitic
granular morphology:
 High-magnifications at 25,000X to
100,000X to characterize the precipitates
in tempered martensite
Research Results
Further Work
Conclusion
As presented hardness results on the table 4 above, this sample 3M11 is fairly
strong and as a matter of fact, of all the samples heated at 300 ◦C,
The microstructure contains of a mixture of martensitic plates and laths.
These martensitic plates have a needle shaped grain structure look. There
is a high growth rate in martensitic transformations resulting in nucleation
becoming the controlling step.
500
520
540
560
580
600
620
640
660
680
700
0 50 100 150 200 250 300 350
HV/KG/MM2
TEMPERING TIMES (MIN)
(300◦C) 3M11-3M18
HV(average)
The softer they get and
hence why the points on the
graph are descending until
they get to a more stable
reading after 30 min.
Decrease in carbon content
of martensite.
681.2
655.4
662.8
649.8
654.6
686.2
3M-WQ 3M-OQ 3M-ACN 3M-AC 3M-OQN 3M-WQN
HV(average) of 300M water, oil and liquid
nitrogen quench.
HV(average)
 water quench followed by
soaking in annealing
gives the highest
hardness
 Low nitrogen pressures
above the oil produce
higher hardnesses and
lower distortions on the
component of alloyed
steels
 Different cooling methods have different effects on hardness
 Tempering also effects the microstructure at elevated temperatures
 In tempering, there are variations of hardness with increasing tempering
time
 The microstructure of a hardened 300M has a very fine ferrite grain size
 The WQN was more consistent with its hardness values so there for I tick
as the most reliable quenching technique.
 Hall-patch strengthening is a one of the methods that are used to
reinforce materials
 Hall-patch describes the yield strength of a material would increase
proportional to square-root of the grain size.
WQN was more
consistent with its
hardness values so
there for I tick as the
most reliable
quenching technique.
400
420
440
460
480
500
520
540
560
580
600
0 10 20 30 40 50 60 70
HV/KG/MM2
TEMPERING TIME
500◦(3M71 - 3M74)
At temperatures that are
above 500◦C the
precipitates start to
coarsen and became
larger. With the lowest
hardness value at the
highest temperature also
proves the higher the
temperature the softer the
material gets