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Stainless Steels with Biocompatible Properties for Medical Devices
Victor Geantă
1,a
, Ionelia Voiculescu
2,b
, Radu Ștefănoiu
1,c
,
Elena Roxana Rusu
1,d
1
POLITEHNICA University of Bucharest, Materials Science and Engineering Faculty, 313 Splaiul
Independenţei, s.6, 060042 – Bucharest, Romania
2
POLITEHNICA University of Bucharest, Engineering and Management of Technological Systems,
313 Splaiul Independenţei, s.6, 060042 – Bucharest,Romania
1
POLITEHNICA University of Bucharest, Materials Science and Engineering Faculty, 313 Splaiul
Independenţei, s.6, 060042 – Bucharest, Romania
1
POLITEHNICA University of Bucharest, Materials Science and Engineering Faculty, 313 Splaiul
Independenţei, s.6, 060042 – Bucharest, Romania,
a
victorgeanta@yahoo.com,
b
ioneliav@yahoo.co.uk,
c
radustefanoiu@yahoo.com,
d
rusuelenaroxana@yahoo.com
Keywords: stainless steels, medical devices, manufacturing, characterization
Abstract. Stainless steels, commercial as well as with special properties, are the principal metallic
materials used for medical devices manufacturing. Stainless steels for medical devices should have
superior mechanical properties, as: hardness, wear resistance, tensile strength, elongation, fracture
toughness, creep resistance etc. This paper aims to present experimental researches regarding the
obtaining in vacuum arc remelting device (VAR) of austenitic and martensitic stainless steels and
their characterization from microstructure and microhardness point of view.
1. Introduction
The technologic and social development nowadays results in life quality increasing by improving
the health services. Thus needs, in very necessary conditions referring to medical intervention,
usage of biocompatible materials more and more valuable to adjust or diminish the trauma impact
on human body. Biocompatible metallic materials are different in structure and properties, but
limited as diversity. As biocompatible metallic materials stainless steels, titanium alloys, cobalt
chrome alloys and alloys used for dental reconstruction are used. Part of them is in contact with
human body, in the case of surgical application, others are used for metallic materials for realizing
medical devices [1, 2].
Usually, the principally metallic materials used for medical instruments are stainless steels.
Besides biocompatibility, stainless steels for medical device should have remarkable mechanical
properties as: hardness, wear resistance, tensile strength, elongation, fracture toughness, creep
resistance etc. All these properties are obtained by combining the charging materials characteristics
with technologies used for elaboration and with the future processing of stainless steel.
2. Steels for medical devices
For medical devices manufacturing, usually, stainless steels are used, most of them classically
obtained. Approximately 1% from stainless steel production is used in medical purposes, for
instruments or surgical elements, justifying their expansion. In spite of this, there are exception
from this general rule, imposed by specific requirements for corrosion resistance, and also for
inclusions’ quantity and dimensions. The stainless steels family for medical instruments can be
described in a lot of ways but the most appropriate seems to be the one referring to metallographic
structure, meaning: ferritic steels, austenitic steels, martensitic (precipitation hardening) steels and
duplex (mixture of ferrite and austenite). The obtainment of this type of steels is dictated by the
chemical composition (alloying degree) and thermal and mechanical treatments applied [3].
Key Engineering Materials Vol. 583 (2014) pp 9-15
© (2014) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/KEM.583.9
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,
www.ttp.net. (ID: 141.85.6.31-30/08/13,12:02:17)
Alloying elements have, as main goal, increasing the corrosion resistance and improving the
mechanic and physical properties. The alloying elements can be alphageneous (Cr, Mo, Si, Ti, Nb)
increasing the interval of (α) solid solution and gamageneous (C, Ni, Mn, N) increasing the interval
of (γ) solid solution. The stainless steel structure depends in the participation of alphageneous and
gamageneous elements in their composition. Cr and Ni are representative elements for
alphageneous elements, the stainless steel structure being affected by the ration between Cr
equivalence and Ni equivalence: E
Cr
and E
Ni
.
E
Cr
= Cr + Mo + 1,5 Si + 0,5 Nb,
E
Ni
= Ni + 30 C + 0,5 Mn,
where: Cr, Mo, Si, Nb represents the percentage content of alphageneous content;
Ni, C, Mn, N represents the percentage content of gamageneous content.
Considering these relationships the stainless steel structure is the one presented in Schaeffler
diagram (fig. 1) [4].
Fig. 1. Schaeffler diagram for stainless steels [4].
ISO 7153-1/1991 specifies the stainless steel categories used for surgical and dentistry devices
used worldwide and also indication regarding their usage [5]. Thus, the most used stainless steels
for medical devices are austenitic and martensitic stainless steels.
The austenitic stainless steels are characterized by low carbon content (<0.1 %), a content of
12...25 %Cr and 8...30 %Ni, presenting an austenitic stabilization till very low temperatures. These
types of steels reveal high performance but also a high price and they have very good mechanical
properties: corrosion resistance, easy to process by plastic deformation and good welding behavior.
Using the austenitic stainless steels is sometimes limited by their low corrosion resistance under
tension, especially in chloride solutions environments at high temperatures. The austenitic stainless
steels are applicable in medical device with lower corrosion resistance: cannula, dental impression
trays, containers, hypodermic needles, steam sterilizers, storage cupboards and work surfaces,
thoracic retractors etc. [5].
The martensitic stainless steels are characterized by a high content of chrome 12…17 % and
higher carbon content, over 0.1 %C. In some cases, the carbon percentage reaches the values of
0.4…0.5 and rarely 1.0. In order to increase their resistance to hot oxidation the silicon is been
added and in order to improve toughness are alloyed with 2...4 %Ni. Some martensitic stainless
steels contain also titanium. The martensitic stainless steels are widely used for dentistry devices
and surgical ones. These stainless steels can be hardened and tempered by heat treatment. Thus,
they are capable to develop a large series of mechanical properties as high hardness for cutting
tools: scalpels, curettes, chisels, forceps, orthodontic pliers, retractors etc.
10 BiomMedD V
The ferritic stainless steels have a limited application for medical devices, examples being solid
handles for guide pins, tools and clamps. Duplex stainless steels do not have, till this moment, a
significant impact in the medical field.
3. Experimental procedure
The experimental part of the scientifically research consisted in processing and characterization
of four batches of stainless steels from biocompatible ones: two from the austenitic stainless steel
(X5CrNi1810 and X5CrNiMo17122) and two from the martensitic stainless steel (X20CrMo13 and
X35CrMo17) used for obtaining surgical devices. In table 1 the standard chemical composition
imposed for stainless steels are presented [6].
Table 1. Standard chemical compositions imposed for stainless steels used for surgical instruments
(DIN, 1977, 1996c, 1997a; Benjamin and Kirkpatrick, 1980, DIN, 1996c, 1997a)
Steel
class
Chemical composition [%]
C Cr Ni Si Mn P S Mo Fe
X5CrNi1810 – C3/1
Standard ≤0.07 17-20 9-11.5 ≤1.0 ≤0.2 ≤0.045 ≤0.03 - bal.
Imposed <0,02 20 10 0,5 1 ≤0.045 ≤0.03 - bal.
X5CrNiMo17122 – C3/2
Standard ≤0.07 16.5-
18.5
10.5-13.5 ≤1.0 ≤2.0 ≤0.045 ≤0.03 2-2.5 bal.
Imposed 0,02 17 13 0,5 1 ≤0.045 ≤0.03 2.5 bal.
X20CrMo13 – C3/3
Standard 0.15-0.18 12-14 ≤1.0 ≤1.0 ≤1.0 ≤0.045 ≤0.03 0.9-1.3 bal.
Imposed 0,2 13 1 0,5 0,5 ≤0.045 ≤0.03 1,2 bal.
X35CrMo17 – C4
Standard 0.33-0.43 15.5-
17.5
≤1.0 ≤1.0 ≤1.0 ≤0.045 ≤0.03 0.9-1.0 bal.
Imposed 0,3 17 1 0,5 0,5 ≤0.045 ≤0.03 1.2 bal.
3.1. Working equipment
Metallic samples from both stainless steels classes for medical devices were obtained in a
vacuum arc remelting (VAR) model MRF ABJ 900 located in ERAMET Laboratory of Materials
Science and Engineering Faculty from POLITEHNICA University of Bucharest. The device has the
following technical characteristics: melting power – min 55kVA; melting current – min 650 A@
60%DS, tri-phase voltage; maximum temperature – 3700
o
C; maximum level for vacuum obtained
by preliminary vacuum and diffusion pumps: 10
-6
mbar; inert gas feeding system – argon; furnace
chamber – stainless steel, double walls water cooled; copper base plate, water cooled having the
following dimensions: Ф 230 mm x 13 mm (thickness); non – feeding electrode made from
tungsten with Ф 65, mm.
3.2. Materials
For stainless steel processing were used a clean charge (raw materials), with high preparation
quality and low content of P and S, having in view that the VAR technology cannot allow the
development of dephosphorization and desulphurization processes. The charge is composed by
materials with high purity: ARMCO Iron, metallic Cr (99.5%), electrolytic Ni (briquettes), metallic
Mo (99.8%), metallic Mn, metallic Si, Al, Graphite etc. For the charge calculus the theoretical
chemical elements assimilation degree into the melt was considered and also the eventual losses by
Key Engineering Materials Vol. 583 11
vaporizing during the vacuum process developing. Thus, for charge composition the following
assimilation rates were considered: η
C
= 98 - 99 %; η
Cr
= 98 – 99.5 %; η
Mo
= 98 – 99.5 %; η
Ni
= 99
– 99.5 %; η
Si
= 98 - 99 %; η
Fe
= 99 %.
The charge calculus for the four charges is shown in table 2.
Table 2. Charge calculus
Charge Material, [g] Mass
[g]
Drawing
out [g]
ARMCO
Iron
Graph
Metallic
Cr
Electrolytic
Ni
Metallic
Si
Metallic
Mn
Metallic
Mo
C3/1 20.5 - 6 3 0.2 0.4 0.2 30.3 30.2
C3/2 19.8 - 5.1 4.2 0.2 0.3 0.7 30.3 30.2
C3/3 25 0.1 4.0 0.3 0.2 0.2 0.4 30.2 30.1
C4 47.6 0.2 10.2 0.6 0.4 0.4 0.8 60.1 59.8
3.3. Working procedure
The working procedure was the classical approach for stainless steels processing in VAR
devices, taking into account the vacuum effect on the charge elements vaporization [6]. The
working conditions have been similar to both alloys types, meaning reaching a minimum pressure
of 10
-4
mbar inside the working chamber, followed by filling with 99,999% Ar, providing the
oxygen content inside the working chamber with a value of 60 ppm. The melting power was
minimum 55 kVA and the melting current was minimum 650A@60%DS tri-phase voltage.
For processing these stainless steel charges were established an electrical, thermal and pressure
regime in order to provide the best alloying elements assimilation in the desired composition gap.
Melting was realized with the aid of electric arch between the tungsten electrode and metallic
charge placed in different measures cavities, existent in the water cooled base copper plate. In order
to ensure the metallic alloys highest homogeneity, the charge was 5 – 7 times remelted, with
successive twists of the ingots formed in each cavity after each remelting.
4. Results and discussions
After the processing and refining of steel in VAR device and after solidification was obtained 4
ingots with approximately same masses for the two steels’ types (fig. 2).
Fig. 2. Stainless steel ingots obtained after solidification.
4.1. Chemical composition
The chemical composition for the experimental samples from stainless steels, determined with
the aid of a spark optical emission spectrometer – SPECTROMAXxM, is shown in table 3.
12 BiomMedD V
Table 3. Chemical composition of stainless steels samples
Sample Chemical composition, [%]
C Si Mn P S Cr Ni Mo Al other
elem.
Fe
C3/1 0.01 1.04 1.14 0.01 0.01 20.40 9.55 0.88 0.06 0.10 66.8
C3/2 0.02 0.60 1.15 0.01 0.01 17.42 12.70 2.43 0.20 0.66 64.8
C3/3 0.15 0.60 0.64 0.01 0.01 13.36 1.15 1.21 0.03 2.24 80.6
C4 0.28 0.56 0.63 0.01 0.01 17.72 1.20 1.15 0.01 0.23 78.2
In the table 3 “other elements” means that in stainless steel chemical composition are find
residual elements as Bi, Sn, Mg, Sb, Cd etc., having no influence on the steel properties. Also, the
presence of aluminum indicates the fact that steel was deoxidized by precipitation.
Analyzing the experimental results obtained provides a very good control for alloying elements.
The charge dosage was correctly calculated, the alloying order well established and thermal and
electrical parameters well chosen. The processing technique was correctly driven providing in this
way the desired compositional gap. The average drawing out, computed as a ratio between the mean
mass value of stainless steel ingots and mean mass value for the charge was over 99% (table 2).
Thus shows the fact that, during processing it was looses just a small amount of alloying elements.
Thus shows that, during technological process development there are minimal losses for charging
elements, due to vaporization under electric arc. The difference is given by the fact that, sometimes,
during the technological process development, drops from the metallic melt are pushed inside the
working space.
4.2. Preparing of samples
To measure the microhardness and for metallographic analysis the casted stainless steel samples
were prepared in LAMET Laboratory of Engineering and Management of Technological Systems
Faculty from POLITEHNICA University of Bucharest in accordance with the following procedure
[8]:
- Cutting, using a high precision cutting machine IsoMet Buehler 4000;
- Embedding in phenol resin, using an automatic press IPA 40;
- Surface polishing, using a Alpha Beta Vector Buehler Polisher;
- Micro etching, using a special electrolytic reagent oxalic acid 10%.
4.3 The metallographic analysis of the samples
The obtained samples have been metallographic studied by optical microscopy, using an inverse
metallographic microscope type Olympus GX51. The results are presented in fig. 3 – 6.
4.4. Microhardness measurement
Were performed 5 successive measurements of microhardness with the aid of a Shimadzu HMV
2TE microdurimeter on the samples’ central zones and edge. The microhardness tests are shown in
table 4.
The samples of austenitic and martensitic stainless steels in cast state show average values for
microhardness inside the interval 278 – 343HV
0.2
. In fact, the hardness in central zone was higher
than on the edge zone due to the cooling conditions. Maximum microhardness was for sample C3/3
– martensitic stainless steel, value being explained by the steel’s higher alloying degree.
Key Engineering Materials Vol. 583 13
Fig. 3. Optical microscopy image for sample C3/1
(Dendrite microstructure with oriented dendrites, with
branches, surrounded by complex eutectic (with
needle compounds separates)).
Fig. 4. Optical microscopy image for sample C3/2
Eutectic matrix with needle and lenses compounds.
Fig. 5. Optical microscopy image for sample C3/3
(Microstructure with prevails dendrites. In
interdendritic spaces there are segregation of lens
compounds and needle eutectic).
Fig. 6. Optical microscopy image for sample C4.
(Microstructure with dendrites and needle eutectic
5 – 10% with precipitation of needle and lenses
compounds and interdendritic microsrinkage).
Table 4. Microhardness of stainless steel samples
Sample Microhardness HV
0.2
Central zone Average
value
Edge zone Average
value
C3/1 308; 292; 267; 271; 255 278 261; 279; 264; 285; 231 264
C3/2 329; 292; 314; 331; 287 311 266; 262; 248; 232; 241 250
C3/3 364; 339; 332; 349; 333 343 324; 336; 311; 333; 354 332
C4 313; 303; 287; 264; 315 296 280; 328; 308; 327; 309 310
5. Conclusions
The experimental researches aimed the obtaining in vacuum arc remelting device austenitic and
martensitic stainless steel and characterization of the obtained samples from microstructure and
microhardness point of view.
Analyzing the obtaining process in vacuum arc remelting device results a very accurate control
for alloying elements. The charge dosage was correctly calculated, alloying order well established.
The processing route was driven correctly, thus resulting in the desired steel compositional gap. The
average drawing out, computed as a ratio between the ingot mass average value and charge mass
average value was over 99 %. Thus highlight the fact that during processing, the losses are very low
due to the vaporization under electric arch. The difference is given by the fact that, during the
technological process, small drops tend to spread inside the working space.
From microstructural point of view, the analyzed samples present dendrites structures, with
oriented dendrites and precipitation of needle and lenses like compounds.
14 BiomMedD V
The microhardness analysis on austenitic and martensitic stainless steel samples, certify that the
microhardness values are in the range of 278 – 343 HV
0.2
. The hardness in central zone was higher
than on the edge zone due to the cooling conditions. It can be stated that the steel alloying degree is
responsible for the resulted values of microhardness.
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