This document discusses using an acetylene and nitrogen gas mixture during turning operations to achieve in-situ surface hardening of AISI 4340 steel. Preliminary results showed the tool-chip interface provided enough energy to decompose the gases and allow carbon diffusion into the work surface, carburizing it and increasing hardness without post-machining heat treatment. An experiment was conducted to optimize cutting parameters like speed, feed, and depth of cut using the gas mixture in order to maximize hardness and minimize surface roughness of the machined steel. Analysis found hardness increased by up to 39.6% with the gas mixture compared to other environments.

1. INTRODUCTION
 A common challenge in die/mold machining of stamping dies,
forming dies, forging dies, injection molds, and blow molds, for example, is
that the hardness of the finished product must be high to achieve long life for
the die/mold. This leads to two options:
(1) machine the die/mold blank in the hardened state, often referred to
as hard machining (turning/milling);
(2) use post-machining heat treatment to realize the required hardness .

2. SURFACE STRENGTHENING
PROCESS

3. PROCESS FLOW OF MANUFACTURING
(a) traditional approach (b) machining hardening
approach

4. VIEW OF CUTTING ZONE
Coaxial pipe
AISI 4340 Steel
(work piece)
Tungsten carbide
(Tool)

5. ABSTRACT
This paper describes the application of a carbonaceous
feed gas (acetylene with a nitrogen shield) in a turning operation to
achieve in situ surface hardening of AISI 4340 steel.
Preliminary results suggest that the tool- chip interface
temperature provides sufficient energy to decompose the feed gas. This
enables carbon diffusion into the work surface which, in effect, carburizes
the surface and increases the hardness during the turning operation.
This could result in the reduction/elimination of post-
machining heat treatment and have significant implications in the die and
mold manufacturing community to improve its productivity.

6. CONT….
An experimental study is done to achieve high hardness in
the machined product and a design of experiment is carried out to find the
cutting parameters like cutting speed, cutting feed & depth of cut and also
to find the best cutting parameters for minimizing the surface roughness
during turning process.
An attempt is also made to analyze the temperature
variation across the tool tip, surface roughness and amount of hardening
for the machined product .The composition of AISI 4340 steel was
analyzed by EDX analysis and the experimental results were analyzed in
MINITAB 16 software.

7. CO AXIAL PIPE

8. TUBE DESIGN
• Gas delivery
system. The inner tube
supplied the acetylene
gas. The outer annular
tube provided the
nitrogen gas shield
• Annular Shielded gas
Delivery
• C2H2 Inner Flow rate.
• N2 Outer Flow rate.

9. CONT…
• The gas was delivered to the system by the concentric tube
assembly.
 The acetylene flow rate (through the inner tube)
was approximately 0.5 liters per minute (LPM) and
 The nitrogen flow rate (through the outer tube)
was approximately 3.5 LPM. The nitrogen shield was
included to minimize oxidation.
 The flow rates were maintained such that the
acetylene gas did not ignite during the cutting operation.

10. CHEMICAL COMPOSITION OF AISI 4340
C Mn Cr Mo Ni Si Fe
0.4 % 0.70 % 0.8 % 0.25 % 1.85 % 0.25 % Balance

11. Levels of the Factor
Cutting
parameters Notations Units
Levels of factor
1 2 3
Cutting speed V m/min 30 60 90
Feed F mm/rev 0.25 0.30 0.35
Depth of cut D mm 0.5 1.0 1.5

12. L9 Orthoganal Array
Expt.No Speed (m/min)
Feed
(mm/rev)
Depth of cut
(mm)
1 1 1 1
2 1 2 2
3 1 3 3
4 2 1 2
5 2 2 3
6 2 3 1
7 3 1 3
8 3 2 1
9 3 3 2

13. TYPES OF GASEOUS ENVIRONMENTS
• Combined C2H2 + N2 environment
• N2 environment
• Atmospheric Air
• Number of Environments = 3
• Total number of Iterations
= No. of Environment × No. of Experiments
= 3 × 9
= 27

14. AISI 4340 STEEL MATERIAL
1 2
3 4 5
6 7 8
10 11 12 13 14
15 17 18 19 20
23 24 25 26 27
21 22
16
9

18. Hardness of the Machined
Product
RUN
NO.
Parameter
level
Experimental results
BHN Values
V F D
Lab Air (A) N2 (B) C2H2 + N2(C)
1 1 1 1 A1 142.63 B1 162.85 C1 187.33
2 1 2 2 A2 145.75 B2 166.61 C2 191.91
3 1 3 3 A3 148.96 B3 166.61 C3 196.65
4 2 1 2 A4 148.96 B4 162.85 C4 187.33
5 2 2 3 A5 152.27 B5 178.63 C5 191.91
6 2 3 1 A6 148.96 B6 182.91 C6 196.65
7 3 1 3 A7 152.27 B7 182.91 C7 206.63
8 3 2 1 A8 152.27 B8 187.33 C8 211.89
9 3 3 2 A9 155.69 B9 191.91 C9 217.35

19. % of Hardness Variation
Trial % (N2, Atm. air) %(N2, C2H2+N2) % (C2H2+N2, Atm. air)
1 A1- B1 14.18 B1- C1 15.03 A1- C1 31.34
2 A2- B2 14.31 B2- C2 15.19 A2- C2 31.67
3 A3- B3 11.85 B3- C3 18.03 A3- C3 32.01
4 A4- B4 9.33 B4- C4 15.03 A4- C4 25.76
5 A5- B5 17.31 B5- C5 7.43 A5- C5 26.03
6 A6- B6 22.79 B6- C6 7.51 A6- C6 32.01
7 A7- B7 20.12 B7- C7 12.99 A7- C7 35.70
8 A8- B8 23.02 B8- C8 13.11 A8- C8 39.15
9 A9- B9 23.26 B9- C9 13.26 A9- C9 39.60

20. ST 20 PRO (Infrared Thermometer)

21. Temperature variation across the
tool tip
RUN
NO.
Parameter
level
Experimental results
Temperature (oC)
V F D
Lab Air (A) N2 (B) C2H2 + N2 (C)
1 1 1 1 A1 280 B1 225 C1 213
2 1 2 2 A2 273 B2 213 C2 217
3 1 3 3 A3 279 B3 226 C3 205
4 2 1 2 A4 265 B4 248 C4 215
5 2 2 3 A5 310 B5 251 C5 237
6 2 3 1 A6 298 B6 267 C6 233
7 3 1 3 A7 335 B7 271 C7 263
8 3 2 1 A8 326 B8 278 C8 255
9 3 3 2 A9 349 B9 293 C9 280

22. Surface Roughness Testing Setup

23. Surface Roughness (Ra)
of the Machined Product
RUN
NO.
Parameter
level
Experimental results
Surface Roughness Values (Ra)
V F D
Lab Air (A) N2 (B) C2H2 + N2(C)
1 1 1 1 A1 5.924 B1 6.815 C1 5.276
2 1 2 2 A2 8.972 B2 8.861 C2 7.594
3 1 3 3 A3 8.874 B3 10.844 C3 8.412
4 2 1 2 A4 8.760 B4 7.484 C4 7.272
5 2 2 3 A5 7.441 B5 5.837 C5 7.210
6 2 3 1 A6 7.264 B6 5.600 C6 5.940
7 3 1 3 A7 6.439 B7 6.053 C7 4.032
8 3 2 1 A8 4.116 B8 4.173 C8 4.067
9 3 3 2 A9 4.237 B9 3.707 C9 3.513

24. Temperature Variation

25. Hardness Variation

27. Micro Structure of AISI 4340 steel

28. Micro Structure for C2H2 + N2
Environment Machined Surface

30. MINITAB OUTPUTS
Signal to noise
ratio
Standard
deviation
Mean
44.1507 22.3838 164.270
44.3474 23.1156 168.090
44.4794 24.1118 170.740
44.3074 19.4270 166.380
44.7014 20.1765 174.270
44.7359 24.5483 176.173
44.9286 27.2533 180.603
45.0446 29.9637 183.830
45.2493 30.9867 188.317

31. Average SN ratio and Rank
Level Speed feed DOC
1 44.33 44.46 44.64
2 44.58 44.70 44.63
3 45.07 44.82 44.70
Delta R 0.75 0.36 0.07
Rank 1 2 3
90
60
30
45.00
44.85
44.70
44.55
44.40
0.35
0.30
0.25
1.5
1.0
0.5
45.00
44.85
44.70
44.55
44.40
Speed
Mean
of
SN
ratios
Feed
Depth of cut
Main Effects Plot for SN ratios
Data Means
Signal-to-noise: Larger is better

32. INTERACTION PLOT FOR SIGNAL TO
NOISE RATIO (SPEED & FEED)
0.35
0.30
0.25
45.2
44.8
44.4
44.0
90
60
30
45.2
44.8
44.4
44.0
Speed
Feed
30
60
90
Speed
0.25
0.30
0.35
Feed
Interaction Plot for SNRA1
Data Means

33. INTERACTION PLOT FOR SIGNAL TO
NOISE RATIO (FEED & DEPTH OF CUT)
1.5
1.0
0.5
45.2
44.8
44.4
44.0
0.35
0.30
0.25
45.2
44.8
44.4
44.0
Feed
Depth of cut
0.25
0.30
0.35
Feed
0.5
1.0
1.5
of cut
Depth
Interaction Plot for SNRA1
Data Means

34. INTERACTION PLOT FOR SIGNAL TO
NOISE RATIO (SPEED & DEPTH OF CUT)
1.5
1.0
0.5
45.2
44.8
44.4
44.0
90
60
30
45.2
44.8
44.4
44.0
Speed
Depth of cut
30
60
90
Speed
0.5
1.0
1.5
of cut
Depth
Interaction Plot for SNRA1
Data Means

35. INTERACTION PLOT FOR MEANS
(SPEED, FEED & DEPTH OF CUT)
190
180
170
1.5
1.0
0.5
0.35
0.30
0.25
190
180
170
90
60
30
190
180
170
Speed
Feed
Depth of cut
30
60
90
Speed
0.25
0.30
0.35
Feed
0.5
1.0
1.5
of cut
Depth
Interaction Plot for MEAN1
Data Means

37. EDAX Analysis
(For V= 90 m/min , F=0.25mm/rev & D=1.5mm)
Near the Center Near the Top

38. Elemental Results
Near the Center
Element Weight% Atomic%
C K 15.74 41.75
Cr K 4.89 5.77
Fe K 71.71 47.69
Mo K 7.66 4.79
Totals 100 100
Near the Top
Element Weight% Atomic%
C K 32.32 64.62
Cr K 4.09 3.64
Fe K 58.71 29.43
Mo K 4.89 2.3
Totals 100 100

39. EDAX Analysis
(For V= 90 m/min , F=0.3mm/rev & D=1.0mm)
Near the Center Near the Top

40. Elemental Results
Near the Center
Element Weight% Atomic%
C K 15.74 41.75
Cr K 4.89 5.77
Fe K 71.71 47.69
Mo K 7.66 4.79
Totals 100.00 100.00
Near the Top
Element Weight% Atomic%
C K 32.32 64.62
Cr K 4.09 3.64
Fe K 58.71 29.43
Mo K 4.89 2.30
Totals 100 100

42. CONCLUSION
• The surface hardness was successfully improved by a in-situ approach
during turning operation.
• Brinell hardness showed an improvement of hardness by 39.6% at cutting
speed of 90m/min. feed of 0.35mm/rev and depth of cut of 1mm in
C2H2+N2 Environment. In almost all the cases the hardness was increased
by more than 25%.
• It was found that Speed is the prominent factor that is effecting hardness i.e.
variation in speed value will affect the hardness by high value when
compared to depth of cut and feed.

43. Contd….
• From main plots it was observed that high speed and feed improve the
surface hardness whereas change in depth of cut has no significant effect
on hardness, and to identify the environment that improves the hardness
to much high value and it was found that C2H2+N2 environment is
dominant over Nitrogen environment.
• Parametric optimization showed that high speed, high feed at any value
of depth of cut will results in high hardness value of AISI 4340 steel in
C2H2+N2 environment.

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45. Cont….
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46. Cont….
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47. Cont….
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