Role of Microstructure on Hardness

 

Role of Microstructure on Hardness

  Key words- Selective laser melting (SLM), 316L Stainless steel (316L SS), Cold spraying (CS), and Cold rolled (CR).

 I.    INTRODUCTION

The Hardness is the measure of a material’s resistance to deformation by surface indentation or by abrasion, some even say it is the quantity that describes strength & heat treatment, some say it is the resistance to permeant deformation but here I will be taking hardness as the resistance to surface indentation. 

So here I will be discussing various ways to increase hardness by changing the microstructure of the material, which can be said as “how the microstructure will effect the hardness of the material”.

The major focus will be on 316L stainless steel manufactured by Selective Laser Melting (SLM) the hardness of the this steel (316LSS by SLM) is compared with conventional manufacturing (casting, cold rolled ) of 316L SS. 

At last how the usage of this 316L SS by SLM in nuclear plant has effects on the microstructure leading to change in hardness of the material is also explored.

II.    Methods

 

A.    Fixed parameter (SLM)

As I already said that major focus will be on 316L SS manufactured by SLM as we all know that there are so many parameters in SLM (laser speed, scanning directions, etc.) for better understanding first let’s see at a particular process parameter i.e.  Speed is 800 mm/s, line spacing 0.1mm (width), thickness of each layer was 0.02 mm, scanning direction is changed with 60⁰.

 

So the parameters are fixed, now a cuboid of 5×5×2 mm is built using the powders of 316L stainless steel. The alloy compositions of the 316L SS is showed in table 1. 

Just see the values of Si, Mo we will see how this will play a major role to achieve our main target i.e. to increase the hardness.  

Table 1

Overall chemical composition of the 316L powder (in wt%).

C	Mn	P	Cr	Ni	Mo	Si	O	N	Fe
0.01	0.98	0.02	17	10.6	2.3	0.4	0.05	0.15	Bal

 Now after manufacturing i.e. both SLM and casted 316L SS they were tested for their hardness but here there is small correction. The size of sample is very small i.e. 5×5×2 mm but the indenter which we will use is be large so the results will give a very false values, so for this conditions there is another test called micro hardness test which is meant to test the hardness for the dimensions of some few mm. so here Microhardness measurement was carried by Zwick/Roell ZHV indenter under a load of 9.8 N for 10s. For better statistical purpose the average of 10 indents was taken. 

Now after testing the Microhardness of SLM 316L SS is 1.5 times higher the Casted 316L SS so now you may get doubt same alloy but a high difference in value so you may think this may be due to any phase transformation but no there is no phase formation.

 So to get the answer we need to look deep into its microstructure. OM, SEM, TEM, XRD, etc. where used to get clean sharp image of the microstructure.

 Now after doing this (XRD, TEM) it is found out that the microstructure in SLM 316L SS is highly different then the Casted 316L SS. In SLM 316L SS the microstructure was found to be a fine & complex columnar structure and major feature is there are formation of sub-grain i.e. a sub crystal inside a grain having its own grain boundary. This sub-grains are much smaller than what normally observed in steel manufactured in casting process. In casting process the microstructure is in the form of large columnar grains with size in between 20 to 40 µm. In SLM the size was around 300nm - 1 µm. The figure 1 gives the best picture of microstructure of the SLM 316L SS.

Figure 1.

 The major reason for the formation of this is very simple in SLM the heat source is acted at very localized point so a small volume gets heated up to a large temperature but environment is at room temperature so there is very large thermal gradient so there is very high potential for heat to transfer at high rate so the Colling rate is drastically very high (10⁶K -10⁸K s^-1) which cannot be achieved in conventional manufacturing. So this leads to high Undercooling temperature.

 To explain this in mathematically we can say if thermal gradient in the liquid (G), and the solidification front growth rate (R) are the main things which helps to understand mathematically this phenomena G/R ratio explains the morphology of the atom while the product G × R determines the cooling rate.

So this leads to have very high nucleation rate as there is no time for diffusion there is no dendrite formation in microstructure and this high nucleation rate also led to the formation of sub grains.

 Now you can see that there is no diffusion so of course the large atom i.e. Mo cannot go inside the grains as it is a large atom it needs some time to move but due to no time it just stays there so this Mo was found to be in grain boundaries.

Now let us see about Si, in general silicon reacts with oxygen but at high temperature it becomes more active and gets more attracted to oxygen so during melting the silicon reacts with oxygen, now we all know SiO₂ can be formed into a glass by Colling at 1000 K/s. So we can say SiO₂ will easily form glass at normal cooling rate but the Colling rate in SLM is 10⁶K -10⁸K s^-1 so this makes easier for SiO₂ to form glass. Now you can see that there is glass phase in the matrix of 316L SS when observed in microstructure this glass was in the shape of circular (2D) so strictly speaking this silicon glass is in the shape of a sphere in the metal matrix. Now why it is forming sphere is mainly due to the fact that it wants to reduce the surface tension so it forms into a sphere. This can be clearly visualized from the figure 2.

 

Figure2.

 

The bonding is also observed to be strong with no cracks in between the glass and steel.

 

We have seen Mo and Si but what about the Mn, Cu , Ni. This elements i.e. Mn, Ni, Cu are in austenite phase and they are called as austenite stabilizers. We all know that austenite cannot be stable in room temperature but it is present in the microstructure of SLM 316L SS so we can see that it is not converting into martensite this can be explained from figure3 & figure 4.

 

 This carbon percentage is very low 0.01% C so it must be very soft but it is (SLM 316L SS) is harder than compared to ferrite this due to the fact that in between A3 and A2 (figure 4) from that temperature it is brought directly to room temperature  

So there is no time for formation of pearlite. Only the austenite and ferrite is remained in the microstructure.

 

Mo also acts as stabilizer for the ferrite region

      

Figure3.

 

Figure4.

But now you may be thinking that when we quench the austenite we get martensite due to stress i.e. due to shear stress. Then at room temperature it should also under go stress induced austenite to martensite transformation but this not happening here.

This because due to high Colling rate there is high nucleation this led to formation of small grains. This extremely fine microstructure increases the yield stress of the element so which will prevents the transformation of austenite to martensite due to stress.

 

There is another feature also i.e the oxygen which was there in the powders or gets taken during the process was removed by the formation of an amorphous chromium-containing silicate.

 

Now to conclude this microstructure of SLM 316L SS I can say that the microstructure contains columnar grains, sub-grains leading to high amount of grain boundaries and Nano particles (Silicon glass).

 

So this all features have a very good impact on dislocation. We know that in general austenite is soft but due the unique microstructure of SLM 316L SS and the Nano particles (Silicon glass) in it try to prevent the formation of new dislocations during indentation so when there is high resistance to formation of new dislocations it will be difficult for the indenter to create more dislocation so more force must be applied to get the required indentation. In this the material becomes Hard. Mission accomplished.     

 

B.    Varying parameter

 

SLM process has large set of parameters (speed, build height, scanning direction, etc ) now we may thinking ok by using SLM 316L SS I can achieve a good hardness but to get this do I need to use the before mentioned parameters only so this will make us not to explore other parameters which have their own advantage and disadvantages.

 

So to know this i.e. is there any effect of parameters on hardness of the 316L SS was done.

The parameters which were varied are hatch space (i.e. how will the laser moves in each plane), building direction, energy density. The samples were prepared and they were analyzed for their microstructure and hardness property.

 The microstructure remained same with not much any special features then compared to above discussion but when the energy density was very high there was a small change in microstructure on surface of the bottom and top surface of the object.

The sub-grains were less in the top surface when compared to bottom surface this because the bottom surface is attached to plate which is at room temperature where as the top surface is formed on a just formed layer which is still hot.

The bottom surface has very high temperature gradient but the top surface has less temperature gradient so due to this the driving force for the nucleation is reduced leading to have a less sub grains in the top layer leading to a fall in Microhardness compared to bottom surface. But it is still hard when compared to the casted 316L SS

 So at last we can say that the change in parameters has impacts on the hardness but not the values of hardness is not drastically varying. 

This problem can be solved by giving special features to the object which will help in better heat transfer rate.

 

C.    Cold spraying on SLM 316L SS

 

By the definition of hardens we can see that it is like a surface property so what if we make new layer of 316L SS on a SLM 316L SS so that we can have a very strong surface.

 

So to achieve this there is process called Cold Spraying

What we will be doing in this that first we manufacture the required object using SLM then by using small powders of 316L SS which are cold sprayed on the surface.

 

We already know the microstructure of the SLM 316L SS but for the cold sprayed it has the same microstructure of the powder there is no special change in the microstructure but hardness value of the Cold sprayed parts increased when comparing to the SLM 316L SS parts. But this was mainly due the plastic deformation and work hardening of Cold sprayed particle.

 

D.    Heat treatment effects on microstructure

 

SLM 316L SS where heat treated but this heat treatment has reduced the hardness of the object because the microstructure started to change mainly losing its sub-grain feature leading to decrease in the hardness of the SLM 316L SS.

 

E.    Effect of Radiation on microstructure and hardness

 

316L SS has a very good usage in nuclear industry now major problem is that temperature is high and there is also irradiation so  this will ultimately effect the microstructure of the 316L SS

So let’s see how the microstructure gets effected leading to change in Microhardness that too on a 316L SS manufactured by SLM process.

 

The experiment was conducted at 350⁰C with 5 MeV Xe²³⁺ ions at a flux of approximately 1.139 ×10⁸ ions/(cm².s)  in a vacuum  so now after testing and doing all the required tests so that we can get a clear and neat picture of microstructure let it be OM, SEM, TEM the following are found out

One thing is that there is no phase transition i.e. the temperature (350⁰C) did not lead or cause any phase changes

 But what is use of 316L SS manufactured by SLM if don’t compare with conventionally manufactured so for comparison an cold rolled (CR) 316L SS was used when the same test condition was applied then there was phase change in CR 316L SS. Now you may be thinking that why this happens in CR 316L SS not in SLM 316L SS

This mainly because the CR 316L SS wants to reduce the stress which caused by radiation so it became into BCC from FCC. The SLM 316L SS did not undergo any phase transformation because of its sub-grains which provided a very resistance to the stress induced by the radiation. But you can say that it is good and evil. Good because there is no phase transformation but the SLM 316L SS has an FCC but CR 316L SS has BCC has we all know that BCC has few interstitial sites compared to FCC so due to this small feature there Is more swelling which caused due to this radiation in 316L SS manufactured by SLM then compared to CR 316L SS.

So there is very high swelling in SLM 316L SS compared to CR 316L SS.

But coming to our major interest i.e. hardening. The value is less in SLM 316L SS then compared to the CR 316L SS this mainly due to the fact this induced radiation as led to formation of Martensite in CR316L SS.  So this martensite formation in the microstructure led to increase in hardness of the CR 316L SS.

III.    Conclusions

 

Microstructure plays a major role in defining the mechanical property of any material so from our discussions we can see that without any edition of the new element just by changing the manufacturing process we can bring a great change in the hardness property.

The effect of radiation environment is also seen on the microstructure and hardness of the material.

IV.    Future Scope

More fabrication methods can be used to get the required property like Selective laser sintering, Directed Energy depositions. The advent of AM process allows for fabrication of complex objects and high cooling rate can lead to have great changes in microstructure so both of the advantages will help to produce great materials with a mind-blowing properties.

V.       Suggestions

The process parameters in Additive manufacturing can be set in a way we can have bulk metallic glass at some points and normal metal at other points so it will be like matrix of same mixture but some points there is glass some points there is metal.

 

References

 

[1]     Saeidi, K. et al. “Hardened austenite steel with columnar sub-grain structure formed by laser melting.” Materials Science and Engineering A-structural Materials Properties Microstructure and Processing 625 (2015): 221-229.

 

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[3]     P. Bajaj, A. Hariharan, A. Kini, P. Kürnsteiner, D. Raabe, E.A. Jägle, Steels in additive manufacturing: A review of their microstructure and properties, Materials Science and Engineering: A,Volume 772,2020,138633,ISSN 0921-5093.

 

[4]     Liverani, Erica & Toschi, Stefania & Ceschini, Lorella & Fortunato, Alessandro. (2017). Effect of Selective Laser Melting (SLM) process parameters on microstructure and mechanical properties of 316L austenitic stainless steel. Journal of Materials Processing Technology. 249. 10.1016/j.jmatprotec.2017.05.042.

 

[5]     Lin, Jiwei & Chen, Feida & Tang, Xiaobin & Liu, Jian & Shen, Shangkun & Ge, Guojia. (2020). Radiation-induced Swelling and Hardening of 316L Stainless Steel Fabricated by Selected Laser Melting. Vacuum. 174. 109183. 10.1016/j.vacuum.2020.109183.

 

[6]     W.D.Callister, Materials Science and Engineering:An Introduction,Wiley,NewYork,2000.