Production and aging of highly porous 17-4 PH stainless steel

Ilven Mutlu • Enver Oktay

Published online: 16 July 2011

� Springer Science+Business Media, LLC 2011

Abstract This study describes production of highly por-

ous 17-4 PH stainless steel for biomedical implant appli-

cations by space holder-sintering technique. 17-4 PH

stainless steel powders were mixed with space holder and

then compacted. For designing pore properties, both

spherical and irregular shaped carbamide with different

particle size ranges were used as space holder and removed

by water leaching. Porous 17-4 PH steel specimens were

sintered at 1,260 �C for 40 min. Boron was used as a liquid

phase sintering additive. In addition, sintered specimens

were aged in order to increase mechanical properties.

Specimens were austenitized at 1,050 �C and then quen-

ched. Quenched specimens were aged at times of 1–6 h at

temperatures between 450 and 570 �C. The pore size and

shape of the 17-4 PH stainless steel foams replicated the

initial size and shape of the carbamide particles. This

suggests that pore properties can be designed by using

proper size, shape and content of space holder.

Keywords Porous metal � Implant � Stainless steel �Aging � Sintering

1 Introduction

Highly porous metals (metal foams) are a new class of

engineering materials and widely used in thermal and

sound insulation, lightweight constructions, heat exchange

and filtration. In addition porous metals can be used in

biomedical applications [1–4]. Porous metals are made by a

range of novel processing techniques many still under

development. Powder metallurgy based space holder-sin-

tering technique has been used to manufacture stainless

steel and titanium which having high melting temperatures.

In addition this process produces open-cell porous struc-

ture, which is suitable to biomedical implant applications

[4–8]. Porosity, pore shape and pore size can be controlled

by using space holder. The space holder method involves

mixing of the metal powder with polymeric binder and

space holder, compaction of the mixture, removing the

space holder and then sintering stages.

Several space holders have been used in the past,

including sodium chloride [9], polymers [10], magnesium

[11], ammonium bicarbonate [7, 8], crystalline carbohy-

drate (cane sugar) [12] and carbamide [5–8]. However,

there were drawbacks in the use of such space holder

materials. Sodium chloride could cause undesirable cor-

rosion to base metal, while the removal of polymeric

materials or carbonate releases environmentally dangerous

by-products during the decomposition. The crucial step is

the removal of the space holder and to select a proper space

holder material. Carbamide was successfully used to pro-

duce highly porous steel specimens.

17-4 PH stainless steel is used for applications in the

aerospace, chemical and food processing industries. 17-4

PH stainless steel can also be used as biomaterial. Its

mechanical properties can be improved by aging. Aging

takes place in three steps; solution treatment (austenitiz-

ing), in which the alloy is heated to a relatively high

temperature that allows alloying elements to go into solu-

tion. Quenching, in which the alloy is cooled to create a

supersaturated solid solution. Aging, in which the quen-

ched alloy is heated to an intermediate temperature and

held for a period of time. At aging temperatures the

alloying elements (Cu) form fine precipitate clusters.

I. Mutlu (&) � E. Oktay

Metallurgical and Materials Engineering Department, Istanbul

University, Istanbul, Turkey

e-mail: imutlu@istanbul.edu.tr

123

J Porous Mater (2012) 19:433–440

DOI 10.1007/s10934-011-9491-8

The precipitates hinder the movement of dislocations and

metal resists deformation and becomes harder [13–16]. It is

known that in the early stage of the aging, coherent Cu

clusters with body-centered cubic crystal structure nucleate

and grow in the supersaturated matrix and lose coherency

and become face-centered cubic (fcc) after reaching a

critical size. In general, maximum strength and hardness

can be obtained after aging at 450–520 �C, during which

the precipitation of coherent Cu-rich clusters occurs. Aging

at higher temperatures results in the precipitation of inco-

herent fcc Cu-rich precipitates [15–18].

The requirements for biomaterials are biocompatibility,

corrosion resistance and sufficient strength close to can-

cellous bone. However, traditional implants are dense and

suffer from problems of inadequate mechanical properties.

Thus, there is an increasing demand to develop new bio-

materials in bone tissue engineering. Highly porous

open-cell metals are novel biomaterials in bone tissue

engineering. They exhibit a porous structure mimicking that

of the cancellous bone and simultaneously, provide adjust-

able mechanical properties through altering the porosity.

In this study, 17-4 PH stainless steel foams was manu-

factured using powder metallurgy based space holder-water

leaching technique. This method was preferred because of

produces open-cell porous structure with sufficient porosity

content for biomedical implant applications. Use of metal

foam as implant allows mechanical interlocking of bone

with metal by bone tissue ingrowth into the pores. Addi-

tionally, by adjusting the porosity content, stiffness can be

controlled in order to reduce the stress-shielding effect

between implant and bone, due to the mismatch of Young’s

modulus and strength. Mechanical properties of implant

must fall in the range of natural bone. Mechanical properties

are connected to the density (porosity). In the foams, density

cannot always be varied freely and in order to gain more

control over mechanical properties, aging is desirable. In this

study, the 17-4 PH stainless steel foams were aged at dif-

ferent temperatures and times, in order to obtain mechanical

properties close to cancellous bone (femur or hip). Gulsoy

and German [8] aged 17-4 PH steel foams having 60%

porosity. However, they aged the foams at only one tem-

perature and time. There is no detailed study on aging of

metal foams. 17-4 PH stainless steel foams were produced up

to 60% porosities. There is no study obtained higher than

60% porous 17-4 PH steel. We have increased the porosity

content of the 17-4 PH stainless steel up to 80%, which is

suitable to implant applications. Moreover, the 17-4 PH

stainless steel foams were aged to close the mechanical

properties to cancellous bone. Aging was carried out at

different temperatures and times, in order to determine the

optimum heat treatment conditions. Aged specimens were

compressively tested before and after aging to obtain a better

understanding of compressive behaviours.

2 Experimental

In this study, gas atomized spherical 17-4 PH stainless steel

powder (Carpenter, Sweden) was used for foam produc-

tion. Figure 1 shows SEM image of the 17-4 PH stainless

steel powder. The chemical composition of the 17-4 PH

stainless steel powder was Fe–4.6% Ni, 15.2% Cr, 0.7%

Mo, 0.4% Nb, 4.9% Cu, 1.4% Si, 0.07% C. Particle size

distribution of the 17-4 PH stainless steel powder was

determined by using a Malvern Mastersizer 2000 analyser.

The mean particle size (D50) of steel was 14.6 lm.

Apparent, tap and pycnometer densities of the stainless

steel powder were determined to be 3.59, 4.44 and

7.81 g cm-3 respectively.

As a space holder, carbamide (Merck, Germany) was

chosen for its advantage of ease of removal in water. As

received carbamide particles were crushed and sieved to

obtain the fractions of -10 ? 500, -1,000 ? 710 and

-1,400 ? 1,000 lm with an irregular shape. Spherical

carbamide particles in range of -1,400 ? 1,000 lm were

also used. To facilitate sintering process of 17-4 PH

stainless steel, boron was added to create a liquid boron-

containing eutectic phase (Fe2B ? Fe). For this purpose,

17-4 PH steel powders were mixed with 0.5 wt% boron

powder (Merck, Germany). The binder for green strength

was polyvinylalcohol (PVA), supplied by Merck, Ger-

many. Figure 2 shows the irregular and spherical carbam-

ide particles with different particle sizes.

Firstly, PVA solution (6 wt%) was added to the 17-7 PH

stainless steel powder. PVA solution was consisted of

2.5 wt% PVA and water. 17-4 PH stainless steel powder

was mixed manually with PVA solution. Carbamide was

moistened with water to form sticky surface and then 17-4

PH stainless steel powder was added. Mixing of the 17-4

PH stainless steel and carbamide was performed in a

Fig. 1 SEM image of the 17-4 PH stainless steel powder

434 J Porous Mater (2012) 19:433–440

123

Turbula mixer. The covered carbamide particles then

compacted uniaxially at 180 MPa into cylindrical speci-

mens with a diameter of 12 mm and heights of 17 mm. The

green specimens were immersed in distilled water at room

temperature to leach the carbamide. About *90% of the

carbamide was leached out in *8–12 h, as confirmed by

weighing the specimens before and after the space holder

removal step. Leaching time was depending on the car-

bamide content (desired porosity) and increasing carbam-

ide content was decreased the leaching time. For example,

specimens with *80% carbamide addition (*80% desired

porosity) were leached at *8 h, while specimens with

*40% carbamide addition (*40% desired porosity) were

leached at *12 h. This was attributed to increasing surface

area of carbamide in contact with water. Increasing time

was due to increased length of porous channels through

which the carbamide must travel.

Thermal debinding temperature of the PVA was deter-

mined by using thermogravimetric analysis (TA, SDT

Q600, DSC/TGA device) at a constant heating rate of 5 �C

under N2 atmosphere. Figure 3 shows the thermogravi-

metric analysis (TGA) curve of the PVA. Debinding tem-

perature of PVA was determined as 410 �C according to

TGA curve. The PVA in the green specimens was ther-

mally removed as part of sintering cycle, which consisted

of slowly heating at a ramp rate of 5–410 �C min-1 with a

dwell time of 40 min (debinding stage), followed by

heating at rate of 10 �C min-1 to sintering temperatures.

The effect of boron addition on the liquid formation

temperature of the 17-4 PH steel was determined by using

differential thermal analysis (TA, SDT Q600, DSC/TGA

device). Figure 4 shows DTA curves of the 17-4 PH steel

powder and 0.5 wt% boron added 17-4 PH steel powder.

Sample containing 0.5 wt% boron shows an endothermic

peak at 1,254 �C in the DTA curve indicating liquid for-

mation. Thus 0.5 wt% boron added 17-4 PH stainless steel

specimens were sintered at a 1,260 �C for 40 min, which

was considerably lower than that of boron-free steels.

Boron activates the sintering process of the iron powders

by the formation of the liquid phase, produced by eutectic

reaction, Fe2B ? Fe ? liquid, which occurs at 1,175 �C.

The liquid phase has a very low solubility in iron and

remains as a continuous network between solid grains,

favouring liquid phase sintering [6]. Optimum amount of

boron addition was chosen to be 0.5 wt%. Higher additions

caused an excess amount of liquid phase. On the other

hand, lower boron additions have a negligible effect.

The sintering cycle was performed in high purity H2

(99.999%) atmosphere in a horizontal tube furnace

(Lenton, UK). In the precipitation hardening (aging) pro-

cess, sintered porous steel specimens were austenitized at

1,050 �C in vacuum furnace and then quenched using N2

(6 bar) as a cooling gas in the same furnace. Vacuum level

was 10-3 mbar. Quenched specimens were precipitation

hardened at times of 1–6 h at temperatures between 450

and 570 �C in H2 atmosphere in a tube furnace.

The microstructures and pore morphology of specimens

were examined by scanning electron microscopy (SEM),

Jeol 5600 and Jeol, 6335F FEG-SEM. For microstructural

investigation, porous specimens were moulded, using

Fig. 2 Carbamide particles a spherical shaped, fraction of -1,400

? 1,000 lm and b irregular shaped, fraction of -1,000 ? 710 lm

Fig. 3 TGA curve of the PVA

Fig. 4 DTA curves of the 17-4 PH steel powder and 0.5 wt% boron

added 17-4 PH steel powder

J Porous Mater (2012) 19:433–440 435

123

Stuers, Epovac vacuum impregnation unit, into a resin to

fill the pores. The moulded specimens were grinded, pol-

ished and then etched by Kalling’s reagent, which com-

posed of 2 g CuCl2, 40 mL HCl and 60 mL ethanol. The

microstructures of the specimens were examined using

Nikon, ME600 optical microscope. The SEM images of the

porous specimens were used to determine the mean pore

size, pore distribution and pore shape by the image ana-

lyzer software. The area of each pore was calculated on the

SEM image and mean equivalent spherical diameter as

pore size and mean sphericity as pore shape was deter-

mined by using image analyser software, Clemex Vision,

PE. Mechanical properties were studied by the compres-

sion tests performed on a Zwick-Roell Z050 materials

testing machine. Compression tests were carried out at a

crosshead speed of 0.5 mm min-1. Compressive yield

stress corresponds to first peak at the compressive stress–

strain curves. Young’s modulus was determined from the

slope of linear portion of the elastic region by software

(TestXper). Hardness was measured by a Vickers microh-

ardness tester (Clemex) using test load of 50 gf. Before the

hardness test the porous stainless steel specimens were

moulded into a resin to fill the pores. Total porosities of the

sintered foams were determined from measurements of

weights and dimensions of the specimens (geometrical

method). Open porosity contents were measured by Hg

porosimeter (Quantachrome Poremaster). Surface rough-

ness parameters (average roughness Ra, maximum height

of the profile Rt and average maximum height of the profile

Rz) of the porous specimens were determined using Time,

TR200 surface roughness tester. Externals surface of the

porous specimens was used for roughness measurements.

Biocompatibility of the 17-4 PH stainless steel foams was

studied by XTT in vitro cytotoxicity assay. Cytotoxicity of

sterilized 17-4 PH stainless steel foams were evaluated in

L929 mouse fibroblast cell culture. Positive and negative

control articles were also prepared to verify the proper

functioning of the test system. After incubation, at 37 �C in

a humidified atmosphere, containing 5% CO2, XTT/PMS

labelling solution (final XTT concentration: 0.3 mg mL-1)

was added to cell culture and the culture incubated for

another 3–5 h. Absorbance (optical density) measurements

were performed at 450 nm using a spectrophotometer.

3 Results

17-4 PH stainless steel specimens with *40–80% porosity

were successfully produced by space holder-water leaching

technique. Total porosity of the specimens consists of

*85–92% open and *8–15% closed porosity. Figure 5

shows the sintered porous 17-4 PH stainless steel

specimens.

The pore shape of the sintered 17-4 PH stainless steel

foams replicated the initial shape of the carbamide particles

that were used as space holder. This suggests that pore

structures can be designed by using proper size, shape and

content of space holder (carbamide). Figure 6 shows the

SEM images of the sintered porous 17-4 PH stainless steel

specimens, which were produced using irregular carbamide

in the range of -1,000 ? 710 lm.

Figure 7 shows mean equivalent spherical diameter

(pore size) and mean sphericity distribution of 70% porous

17-4 PH stainless steel specimens, which were produced

using irregular carbamide in the range of -1,000 ?

710 lm. As seen, mean size of the pores was 607 lm,

which is suitable for biomedical implant applications. Pore

morphology is also investigated by the image analyser in

terms of the mean sphericity and found to be 0.57.

Meanwhile, mean equivalent spherical diameter and mean

sphericity of the carbamide particles was determined to be

860 and 0.64 lm respectively. The decrease in the size and

sphericity was attributed to crushing of the carbamide

particles during pressing and moistening before mixing.

Porosity and pore size both play a critical role in bone

growth. The minimum requirement for pore size is con-

sidered to be *100 lm due to cell migration and transport.

Higher porosity and larger pore size result in greater bone

growth. For the pores with size less than 100 lm, cells did

not grow into the pores because of spanning of pores

by cells.

Table 1 shows the mean size of carbamide particles and

mean pore size of the foams. As seen in Table 1, the final

pore size is directly related to the carbamide particle size.

Table 2 shows mean sphericity of carbamide particles and

mean pore sphericity of the foams. Pore shape was also

similar to initial carbamide particle shape, as expected.

The compressive stress–strain curves of sintered 17-4

PH stainless steel specimens with different porosities are

Fig. 5 Sintered porous 17-4 PH stainless steel specimens a 60% and

b 70% porosity

436 J Porous Mater (2012) 19:433–440

123

illustrated in Fig. 8. Three distinct regions characterized

the curves; the initial part is elastic region where yielding

and cell walls bending is observed and stress rising linearly

with strain; a large plateau region at nearly constant

compression stress where cell walls buckle and a densifi-

cation region where the flow stress rapidly increases. The

stress, after a first maximum, drops significantly as a result

of the collapse of a pore layer. The value of this peak

increases with increasing density (decreasing porosity) of

the material. Once the cell edge collapses at the yield point,

the collapsed edge has little ability to bear the load and

bends easily by a low stress. Long plateau regions with

nearly constant flow stresses to large strains are an indi-

cation of open cellular morphology. It is clear that porosity

affects the Young’s modulus and compressive yield

strength of foams. Figure 9 shows the effects of porosity on

the Young’s modulus and compressive yield strength of the

foams.

Figure 10 shows the effect of aging temperature on the

compressive stress–strain curves of the sintered porous

stainless steel specimens. Compressive yield strength of

aged foams was observed to vary between 60 and

110 MPa. The resultant Young’s modulus of the specimens

was between 0.73 and 1.54 GPa. Compressive yield

strength and Young’s modulus of as sintered specimens

before aging were 40 MPa and 0.47 GPa respectively.

Young’s modulus and compressive yield strength values of

cancellous bone are 0.09–1.5 GPa and 40–150 MPa

respectively [4, 19–21]. As a result, mechanical properties

of the aged 17-4 PH stainless steel foams were close to

cancellous bones.

Fig. 6 SEM images of the 17-4 PH steel foams, a inner section

b surface

Fig. 7 a Mean spherical diameter and b mean sphericity distribution

of porous specimens produced by using irregular carbamide particles

in the range of -1,000 ? 710 lm

Table 1 Mean size of carbamide particles and mean pores size of the

foams

Carbamide

particle shape

Carbamide particle

size fraction (lm)

Mean carbamide

particle size (lm)

Mean pore

size (lm)

Irregular -1,400 ? 1,000 1,272 920

Irregular -1,000 ? 710 860 607

Irregular -710 ? 500 580 446

Spherical -1,400 ? 1,000 1,312 895

J Porous Mater (2012) 19:433–440 437

123

Figure 11 shows the effects of aging temperature on the

Young’s modulus and compressive yield strength of the

foams. Young’s modulus and compressive yield strength of

the porous specimens increased with increasing aging

temperature and attain its peak value at 480 �C and then

decrease. At higher temperatures mechanical properties

were decreased from maximum value. Figure 12 shows the

effect of aging time on the Young’s modulus and com-

pressive yield strength. Optimum properties were obtained

at 2 h aging. This phenomenon was attributed to the for-

mation of coherent Cu-rich precipitates in optimum size

with bcc crystal structure and homogeny distribution in the

microstructure at 480 �C for 2 h aging. Hardening is

attributed to pinning of dislocations by precipitated copper

rich clusters. The decrease in mechanical properties with

increasing temperatures above 520 �C was attributed to

formation of coarse and incoherent copper rich precipitates.

When precipitates grow and then lose coherency with

matrix, precipitates lose their precipitation hardening

effect.

Maximum hardness was obtained at 480 �C for 2 h

aging, in which precipitation of coherent copper rich

clusters occur. There was only a slight increase in hardness

at low and high aging temperatures. Aging the 17-4 PH

Table 2 Mean sphericity of carbamide particles and mean pore

sphericity of the foams

Carbamide

particle shape

Carbamide particle

size fraction (lm)

Mean

carbamide

sphericity

Mean pore

sphericity

Irregular -1,400 ? 1,000 0.64 0.57

Irregular -1,000 ? 710 0.62 0.56

Irregular -710 ? 500 0.60 0.53

Spherical -1,400 ? 1,000 0.73 0.69

Fig. 8 Effect of porosity on compressive stress-compressive strain

curves of the sintered 17-4 PH stainless steel

Fig. 9 Effect of porosity on the compressive yield strength and

Young’s modulus of the 70% porous 17-4 PH stainless steel

Fig. 10 Effect of aging temperature on the compressive stress–strain

curves of the sintered porous specimens

Fig. 11 Effect of aging temperature on the Young’s modulus and

compressive yield strength of 70% porous specimens (aging time:

2 h)

438 J Porous Mater (2012) 19:433–440

123

stainless steel at higher temperatures (570 �C) after 4 h led

to a significant decrease in the hardness value subsequent

to the initial rise in hardness during aging. Over-aging

brings about coarse and incoherent precipitates and alters

the mechanical properties.

Figure 13 shows the microstructure of the boron added

17-4 PH stainless steel specimen. Boron addition activates

the sintering process of the iron powders by the formation

of the eutectic liquid phase. Eutectic reaction, Fe2B ?

Fe ? liquid, occurs at *1,175 �C. The liquid phase has a

very low solubility in iron and remains as a continuous

network between solid grains, favouring the liquid phase

sintering. Optimum amount of boron addition was chosen

to be 0.5 wt%. Higher additions caused an excess amount

of liquid phase. On the other hand, lower boron additions

have a negligible effect on sintering process and mechan-

ical properties.

Optic microscope image taken from the cell walls of the

aged porous 17-4 PH stainless steel specimens are shown in

Fig. 14. Microstructure of the aged 17-4 PH stainless steel

consists of lathe martensite matrix and d-ferrite. In addition

micropores at the particle contact areas are visible due to

incomplete sintering. However, very fine copper rich pre-

cipitates in the lathe martensite matrix could not be

observed by optical microscope.

Copper rich precipitates in the aged 17-4 PH stainless

steel foams were observed by FEG-SEM at high magnifi-

cations. Figure 15 shows the FEG-SEM image from cell

walls of aged 17-4 PH stainless steel foams. The fine

copper rich precipitates observed in the lathe martensite

matrix. Copper content of the precipitates was *70%

according to EDS analysis.

Biomedical implant materials are fabricated to have

sufficiently rough surface to increase contact area with

tissues. Cell viability is dependent on roughness of sub-

strate on which they are attached. Arithmetic mean devi-

ation (Ra) of the foams was *7 lm, which is suitable to

implant applications. Rt and Rz values were 8 and 18 lm

respectively. Since the standard deviation of surface

roughness parameters was about 2 lm, the surface rough-

ness parameters of the porous specimens were homoge-

neous. The XTT assay was used to determine in vitro

cytotoxicity of the 17-4 PH stainless steel. It is found that,

17-4 PH stainless steel foams do not have a cytotoxic

potential on cells according to XTT assay. Viability value

of the foam (81%) was close to viability value of negative

control (91%), which means the 17-4 PH stainless steel

foam has no cytotoxic potential. Meanwhile, viability of

the positive control specimen was 4%. Consequently, the

results indicated potential applications of the 17-4 PH

stainless steel foam as biomedical implant material.

Fig. 12 Effects of aging time on Young’s modulus and compressive

yield strength of 70% porous specimens (aging temperature: 480 �C)

Fig. 13 Microstructure of boron added 17-4 PH stainless steel

specimen Fig. 14 Optic microscope image of the aged porous specimen

J Porous Mater (2012) 19:433–440 439

123

4 Conclusions

Highly porous 17-4 PH stainless steel specimens were

manufactured using space holder-water leaching technique.

Carbamide was used as a space holder and was removed by

water leaching at room temperature. The water leaching is

attractive because water is non-toxic, non-flammable and

use of water eliminates environmental drawbacks. The final

porosity was directly related to the added fraction of car-

bamide, as expected. Pore shape was also similar to initial

carbamide particle shape. Mechanical properties were

depending on the density (porosity) of the metal foams.

But, density cannot always be varied freely. In order to

gain more control over the mechanical properties heat

treatment (aging) is essential. The sintered foams were

aged in order to obtain mechanical properties close to

cancellous bone. Porous structure, pore size and the

mechanical properties of the 17-4 PH stainless steel foams

were similar to cancellous bone. As a result space holder-

water leaching technique is suitable to stainless steel foam

production for biomedical implant applications.

Acknowledgments This work was supported by Scientific Research

Projects Coordination Unit of Istanbul University, Project number

T-1430. It was partially based on a Ph.D. thesis pursued by Ilven

Mutlu.

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