Work hardening characteristics of titanium T 160 J Brettle W Osman Introduction As part of a Dept of Health and Social Security contract to investigate the reasons for failure of metallic surgical implants work in the Metallurgy Division of AWRE has included the examination of several failed titanium implants. The work reported here, to determine the relationship between hardness and cold work for coxlmercially pure titanium, was andertaken so that:
Thickness measurements, in
Mean thickness in
Reduction in thickness, %
0.0606 0.0599 0.0602
0.0600 0.0600 0.0601
0.0600 0.0605 0.0605
0.0602
0
0.0545 0.0545 0.0547
0.0546 0.0548 0.0540
0.0544 0.0542 0.0544
0.0544
8
0.051 6 0.0516 0.051 5
0.0520 0.0513 0.0515
0.0514 0.0519 0.051 1
0.051 5
13
0.0472 0.0473 0.0480
0.0483 0.0471 0.0469
0.0471 0.0475 0.0479
0.0474
20
0.0448 0.0440 0.0450
0.0447 0.0444 0.0449
0.0436 0.0446 0.0445
0.0445
0.0422 0.0424 0.0419
0.0425 0.0422 0.0420
0.0420 0.0425 0.0421
0.0389 0.0394 0.0386
0.0392 0.0388 0.0391
0.0388 0.0389 0.0397
0.0356 0.0352 0.0355
0.0361 0.0354 0.0363
0.0354 0.0360 0.0358
0.0331 0.0335 0.0329
0.0335 0.0336 0.0341
0.0332 0.0338 0.0338
0.0334
0.0313 0.0318 0.0308
0.0326 0.0318 0.0330
0.0319 0.0320 0.0315
0.0277 0.0280 0.0280
0.0275 0.0266 0.0290
0.0250 0.0252 0.0249
0.0245 0.0251 0.0251
(a) From hardness measurements on titanium implants the degree of cold working received during fabrication could be estimated. (b) Cold work in titanium specimens, stock bars, etc could be likewise determined by hardness measurements.
Experimental The stock material used was 7/8 in dia hot rolled bar*.
VPN measurements Standard 10 kg load 2/3 in Mean deviation ocular VPN (T 235 249 249
251 254 237
245 227 227
241
10
260
252 266 251
262
10
272 266 279
266 290 279
270 276 274
274
8
283 304 311
274 299 330
258 297 309
296
22
25
309 302 302
299 322 314
302 311 297
306
8
0.0422
29
322 317 314
309 357 325
317 306 325
321
15
0.0390
34
319 311 342
299 319 309
309 322 302
313
13
40
311 342 327
319 319 319
322 330 314
322
44
336 330 309
311 325 314
342 325 314
322
12
0.0318
47
347 317 326
320 341 332
338 338 329
332
7
0.0270 0.0275 0.0291
0.0278
53
354 327 333
333 339 330
351 327 327
335
10
0.0243 0.0254 0.0248
0.0249
58
339 351 348
348 345 351
325 330 342
342
9
270 262
0.0357
Note: Arithmetic means calculated from
Standard deviation, u, calculated from
?h 9
u =
n- 1
Table 1
* Originally as received from IMI Ltd - 1% in dia bar, annealed 675OC air cool. Cast No A6109. Subsequently hot rolled to 7/8 in dia bar and annealed 675OC. air cool. Analysis: 0.035 Fe. Figures for hydrogen, oxygen and nitrogen are not quoted as these may have been significantly affected by the multiple heat treatments the material has undergone. No analysis on the strip after rolling has been carried out but some idea of the hardening due to these interstitial elements can be gained from the hardness figures for the material in the annealed state. Downloaded from eim.sagepub.com at CAMBRIDGE UNIV LIBRARY on June 4, 2016
(Q IMechE 1976 Vol. 5, No. 2
41
After soaking for 1/2 h at 800°C the bar was forged to approximately 1/10 in thick plate in stages with several interstage anneals. After pickling and brightening* 2 jn strips were cut from the plate and cold rolled by hand. Two strips were produced, A and B, most of the hardness measurements were carried out on strip A but strip B was treated exactly the same as A so that duplicate measurements could be made on doubtful results. At this stage vacuum annealing at 675'C for 1/2 h was performed (pressure 0.0005 mmHg) and hardness measurements were taken at nine equally spaced positions on the strip (fig 1). The thickness of the strip was measured with a micrometer at these nine positions and averages taken. After cold rolling by hand through a small pair of bench rolls the process of hardness and thickness measurements was repeated until the strip, originally 0.06 in thick, was reduced to 0.025 in thick, ie, a reduction of thickness of about 60%.Measurements were made at approximately every 5% reduction in thickness. The results are shown in table 1 . At about 60%reduction the strip began t o crack at the edges and rolling was discontinued.
Results and discussion The results are shown in fig 2; the bars are 20 long and indicate the spread in experimental results. The work hardening curve of titanium T160 may be compared with that of AISI 31 6 stainless steel (fig 3)l, another widely used implant material. The hardness of titanium T160 starts at a relatively high value of 240 W N and rises slowly; the high initial hardness is due t o solid solution strengthening by oxygen, nitrogen, carbon and iron additions. The titanium is stable ti phase at room temperature and no transformation takes place on cold working. AISI 316 stainless steel, however, in the annealed state is much *
Pickling solution:
X
X
X
X
X
X
X
X
X
Positions marked
thus:- X
Fig I . Positions of hardness and thickness measurements on titanium strip
softer, about 190 VPN, but work hardens more rapidly to give a hardness of 360 VPN at 30%cold work compared with 3 10 W" for titanium T160. The higher work hardening rate is due in part to austenitic stainless steel having two work hardening mechanisms; in addition to strain hardening
4 vlo Hydrofluoric Acid 20 vlo Hydrochloric Acid 76 vlo Distilled Water
Brightening solution:
2% vlo Hydrofluoric Acid 5 v/o Nitric Acid 96% vlo Distilled Water
Temperature:
25OC I
I
I
I
r
I
I
I
I
---l-1 T
Fig 2. Hardnesshduction in thickness for titanium T160 Downloaded from eim.sagepub.com at CAMBRIDGE UNIV LIBRARY on June 4, 2016
42
Engineering in Medicine
@ IMechE 1976
an austenite to mastensite transformation takes place on deformation giving a contribution from transformation hardening. The effect of cold working on the mechanical properties of titanium is, as one may expect, to increase the UTS and proof stress while lowering the ductility; because of the appreciable drop in ductility titanium is normally used in the annealed condition, strength requirements being met by using a grade containing the appropriate concentration of solution strengthening elements2. The small number of titanium implants examined at AWRE have had hardnesses in the range 240-260 VPN although on occasion hardnesses as low as 220 VPN have been recorded, it must be assumed that these implants were made from softer grades of titanium. A larger number of surgical screws have been examined and the hardness tends to be somewhat higher, ranging from 240 to 280 VPN3. This indicates that the screws suffer more cold working during fabrication than the bone plates.
References
I
I
I
I
I
I
I
I
360 340-
320300-
l =.
280-
260 240-
220-
1. Hughes, A N Private communication. 2. IMI Publication 4Ed/Mk23/13/467. 3. Jordan, B A Private communication.
I
I
5
10
Acknowledgement Copyright 0Controller HMSO, London, 1976.
15 20 Cold work per cent
1
I
25
30
35
Fig 3. Hardness/cold work for AI5I 316 stainless steel
One of the aims of this IMechE conference was to bring together engineers working on both practical and theoretical aspects of mechanisms. Thus the papers cover a wide range including, a t one end of the spectrum, the development of plunging joints for use in automotive transmission systems, to a unified theory for the analysis of single loop spatial mechanisms a t the other. Other subjects covered are: kinematics of a complete arm prosthesis; mechanisms for performing the functions of arms and legs and design of four-bar path generating linkages. This volume will be of interest to designers, development engineers and research workers in the field of mechanisms.
84 pageslA4lhardbound Price f8.00,f0.60postage and packaging. 818.00 US and Canada, plus postage and packaging.
Orders, or for further details, to: Marketing Manager, Mechanical Engineering Publications Ltd, PO Box 24, Northgate Avenue, Bury St Edmunds, Suffolk IP32 6BW, England.
~~~
~~~
Downloaded from eim.sagepub.com at CAMBRIDGE UNIV LIBRARY on June 4, 2016
@ IMechE 1976 Vol. 5, No. 2
43
Thickness measurements, in
Mean thickness in
Reduction in thickness, %
0.0606 0.0599 0.0602
0.0600 0.0600 0.0601
0.0600 0.0605 0.0605
0.0602
0
0.0545 0.0545 0.0547
0.0546 0.0548 0.0540
0.0544 0.0542 0.0544
0.0544
8
0.051 6 0.0516 0.051 5
0.0520 0.0513 0.0515
0.0514 0.0519 0.051 1
0.051 5
13
0.0472 0.0473 0.0480
0.0483 0.0471 0.0469
0.0471 0.0475 0.0479
0.0474
20
0.0448 0.0440 0.0450
0.0447 0.0444 0.0449
0.0436 0.0446 0.0445
0.0445
0.0422 0.0424 0.0419
0.0425 0.0422 0.0420
0.0420 0.0425 0.0421
0.0389 0.0394 0.0386
0.0392 0.0388 0.0391
0.0388 0.0389 0.0397
0.0356 0.0352 0.0355
0.0361 0.0354 0.0363
0.0354 0.0360 0.0358
0.0331 0.0335 0.0329
0.0335 0.0336 0.0341
0.0332 0.0338 0.0338
0.0334
0.0313 0.0318 0.0308
0.0326 0.0318 0.0330
0.0319 0.0320 0.0315
0.0277 0.0280 0.0280
0.0275 0.0266 0.0290
0.0250 0.0252 0.0249
0.0245 0.0251 0.0251
(a) From hardness measurements on titanium implants the degree of cold working received during fabrication could be estimated. (b) Cold work in titanium specimens, stock bars, etc could be likewise determined by hardness measurements.
Experimental The stock material used was 7/8 in dia hot rolled bar*.
VPN measurements Standard 10 kg load 2/3 in Mean deviation ocular VPN (T 235 249 249
251 254 237
245 227 227
241
10
260
252 266 251
262
10
272 266 279
266 290 279
270 276 274
274
8
283 304 311
274 299 330
258 297 309
296
22
25
309 302 302
299 322 314
302 311 297
306
8
0.0422
29
322 317 314
309 357 325
317 306 325
321
15
0.0390
34
319 311 342
299 319 309
309 322 302
313
13
40
311 342 327
319 319 319
322 330 314
322
44
336 330 309
311 325 314
342 325 314
322
12
0.0318
47
347 317 326
320 341 332
338 338 329
332
7
0.0270 0.0275 0.0291
0.0278
53
354 327 333
333 339 330
351 327 327
335
10
0.0243 0.0254 0.0248
0.0249
58
339 351 348
348 345 351
325 330 342
342
9
270 262
0.0357
Note: Arithmetic means calculated from
Standard deviation, u, calculated from
?h 9
u =
n- 1
Table 1
* Originally as received from IMI Ltd - 1% in dia bar, annealed 675OC air cool. Cast No A6109. Subsequently hot rolled to 7/8 in dia bar and annealed 675OC. air cool. Analysis: 0.035 Fe. Figures for hydrogen, oxygen and nitrogen are not quoted as these may have been significantly affected by the multiple heat treatments the material has undergone. No analysis on the strip after rolling has been carried out but some idea of the hardening due to these interstitial elements can be gained from the hardness figures for the material in the annealed state. Downloaded from eim.sagepub.com at CAMBRIDGE UNIV LIBRARY on June 4, 2016
(Q IMechE 1976 Vol. 5, No. 2
41
After soaking for 1/2 h at 800°C the bar was forged to approximately 1/10 in thick plate in stages with several interstage anneals. After pickling and brightening* 2 jn strips were cut from the plate and cold rolled by hand. Two strips were produced, A and B, most of the hardness measurements were carried out on strip A but strip B was treated exactly the same as A so that duplicate measurements could be made on doubtful results. At this stage vacuum annealing at 675'C for 1/2 h was performed (pressure 0.0005 mmHg) and hardness measurements were taken at nine equally spaced positions on the strip (fig 1). The thickness of the strip was measured with a micrometer at these nine positions and averages taken. After cold rolling by hand through a small pair of bench rolls the process of hardness and thickness measurements was repeated until the strip, originally 0.06 in thick, was reduced to 0.025 in thick, ie, a reduction of thickness of about 60%.Measurements were made at approximately every 5% reduction in thickness. The results are shown in table 1 . At about 60%reduction the strip began t o crack at the edges and rolling was discontinued.
Results and discussion The results are shown in fig 2; the bars are 20 long and indicate the spread in experimental results. The work hardening curve of titanium T160 may be compared with that of AISI 31 6 stainless steel (fig 3)l, another widely used implant material. The hardness of titanium T160 starts at a relatively high value of 240 W N and rises slowly; the high initial hardness is due t o solid solution strengthening by oxygen, nitrogen, carbon and iron additions. The titanium is stable ti phase at room temperature and no transformation takes place on cold working. AISI 316 stainless steel, however, in the annealed state is much *
Pickling solution:
X
X
X
X
X
X
X
X
X
Positions marked
thus:- X
Fig I . Positions of hardness and thickness measurements on titanium strip
softer, about 190 VPN, but work hardens more rapidly to give a hardness of 360 VPN at 30%cold work compared with 3 10 W" for titanium T160. The higher work hardening rate is due in part to austenitic stainless steel having two work hardening mechanisms; in addition to strain hardening
4 vlo Hydrofluoric Acid 20 vlo Hydrochloric Acid 76 vlo Distilled Water
Brightening solution:
2% vlo Hydrofluoric Acid 5 v/o Nitric Acid 96% vlo Distilled Water
Temperature:
25OC I
I
I
I
r
I
I
I
I
---l-1 T
Fig 2. Hardnesshduction in thickness for titanium T160 Downloaded from eim.sagepub.com at CAMBRIDGE UNIV LIBRARY on June 4, 2016
42
Engineering in Medicine
@ IMechE 1976
an austenite to mastensite transformation takes place on deformation giving a contribution from transformation hardening. The effect of cold working on the mechanical properties of titanium is, as one may expect, to increase the UTS and proof stress while lowering the ductility; because of the appreciable drop in ductility titanium is normally used in the annealed condition, strength requirements being met by using a grade containing the appropriate concentration of solution strengthening elements2. The small number of titanium implants examined at AWRE have had hardnesses in the range 240-260 VPN although on occasion hardnesses as low as 220 VPN have been recorded, it must be assumed that these implants were made from softer grades of titanium. A larger number of surgical screws have been examined and the hardness tends to be somewhat higher, ranging from 240 to 280 VPN3. This indicates that the screws suffer more cold working during fabrication than the bone plates.
References
I
I
I
I
I
I
I
I
360 340-
320300-
l =.
280-
260 240-
220-
1. Hughes, A N Private communication. 2. IMI Publication 4Ed/Mk23/13/467. 3. Jordan, B A Private communication.
I
I
5
10
Acknowledgement Copyright 0Controller HMSO, London, 1976.
15 20 Cold work per cent
1
I
25
30
35
Fig 3. Hardness/cold work for AI5I 316 stainless steel
One of the aims of this IMechE conference was to bring together engineers working on both practical and theoretical aspects of mechanisms. Thus the papers cover a wide range including, a t one end of the spectrum, the development of plunging joints for use in automotive transmission systems, to a unified theory for the analysis of single loop spatial mechanisms a t the other. Other subjects covered are: kinematics of a complete arm prosthesis; mechanisms for performing the functions of arms and legs and design of four-bar path generating linkages. This volume will be of interest to designers, development engineers and research workers in the field of mechanisms.
84 pageslA4lhardbound Price f8.00,f0.60postage and packaging. 818.00 US and Canada, plus postage and packaging.
Orders, or for further details, to: Marketing Manager, Mechanical Engineering Publications Ltd, PO Box 24, Northgate Avenue, Bury St Edmunds, Suffolk IP32 6BW, England.
~~~
~~~
Downloaded from eim.sagepub.com at CAMBRIDGE UNIV LIBRARY on June 4, 2016
@ IMechE 1976 Vol. 5, No. 2
43
