ITSC 2015- Int. Thermal Spray Conf., May 11-14 2015, California, USA

The Many Facets and Complexities of 316L and the Effect on Properties

Ingrid Hauer Miller Höganäs AB, Höganäs, Sweden state and country

Ingrid.hauer@hoganas.com, +46702066244

Abstract

One of the most widely accepted and versatile materials used

with many coating deposition methods is austenitic stainless

steel 316L. This is due to the combination of good corrosion

and mechanical properties that are suitable for numerous

varying applications. As a result of our familiarity with 316L

we assume the properties achieved will be within the expected

frame as described in the literature. Deposition techniques and

component size and shape have an impact on the heating and

cooling variations which will then affect the microstructure of

the final coating. This in turn will result in variations in the

properties achieved. Some of these numerous factors such as:

deposition technique, component geometry and size, impurities

in the substrate and chemistry of the substrate will be

considered. The influence of these factors on the

microstructure and the definitive properties of 316L will be

presented.

Introduction

Welding provides very high bond strength between deposit

and substrate and can be applied in thickness greater than most

other techniques; typically in the range 2-20 mm. Welding is

often used for resurfacing. These processes involve application

of heat to the processed component and depending on the

material from which it is made and its conditions, certain

precautions may need to be taken. During fusion weld

surfacing, the coating material is raised to its melting point,

which means that metals and alloys used for the purpose must

have a melting point similar or less than the substrate

materials. Exceptions of this general rule are found in friction

surfacing and explosive cladding. Other coating materials with

higher melting points, such as ceramics, may be applied by

thermal spraying processes. The welding methods used in this

investigation are PTA and laser cladding.

Laser cladding utilizes a high power laser beam as a heat

source for the melting and fusing of the substrate and filler

materials. As the laser is a beam of electromagnetic radiation,

all of the injected energy is confined within a small spot with

no diffusion due to convection from hot gasses or

electromagnetic forces taking place. As a result, the heat input

to the part is minimised and higher welding quality and

efficiency is attained. Filler metal is added in many ways with

the most common being powder injection. The creation of a

melt pool where a fine film of base material is melted and the

filler material is injected, results in the formation of a welding

bead which is strongly adhered to the substrate. The dilution in

the deposit is around 5%.

The high intensity of the laser beam and the associated low

heat input results in minimised distortion, residual stresses and

base metal degradation. Furthermore, the higher cooling rates

attained result in finer microstructures and smaller heat

affected zones leading to improved mechanical properties.

These facts allow the processing of critical parts which could

not be repaired in the past with other fuse welding techniques.

This opens new opportunities for performance enhancement

and cost reductions through the salvaging of parts which were

previously scraped. Furthermore, the impeccable control of

overlay thickness and low overlay roughness minimises the

machining allowances and hence lowers the production or

repair costs.

The plasma transferred arc, (PTA) process uses an argon

shielded tungsten arc as the source of energy. The powder is

introduced into a combined arc/plasma stream to form a

molten pool on the workpiece. Additionally, the arc between

the workpiece and gun produces surface melting of base

material and dilution of 5 to 15% in the deposit is typical. This

method should not be confused with plasma spraying which

used a non-transferred arc to generate the heating plasma for

spraying powders.

The main differences between the two methods are:

- Higher heat input in PTA compared to laser, which

will also result in a greater heat affected zone and

higher dilution

- Faster cooling rate for laser which will give a finer

microstructure, resulting in higher hardness and

superior wear resistance.

When the same coating method is used, the final quality of the

clad is depending of several factors, such as: material used for

cladding, component geometry, component size, impurity size

and number.

The purpose of this work is to show how the different factors

influence the final quality of the coating, when the stainless

steel 316L is used.

ITSC 2015- Int. Thermal Spray Conf., May 11-14 2015, California, USA

The differences in the coating properties are also presented

when two different methods are used: laser cladding and

plasma arc welding (PTA).

Experimental Method

The coating material used in this investigation is the austenitic

stainless steel 316L with the composition shown in Table 1.

The surfacing was made using laser cladding and PTA

methods on low alloyed carbon steel substrates.

Table 1: 316L composition.

C% Si% Fe% Cr% Ni% Mo% Mn%

316L <0.03 0.8 base 17 12 2.5 1.5

All tests were performed in the Höganäs laboratory.

The laser cladding was performed on a Coherent direct diode

laser, 4 kW, 808 nm wavelength. Substrate was 1018 steel in 3

forms and the clads were applied in a single layer. Shapes used

are as follows:

- 70 x 100 x 10 mm (w x l x t) plate

- 50 x 200 x 20 mm (w x l x t) plate

- 25 mm ID x 30 mm OD tube

Same parameters were used for all the laser cladding tests:

- Power 3kW

- Cladding speed 5 mm/s

- Feed rate 30 g/min

- Feed gas (Ar) 3 l/min

- Shield gas (Ar) 10 l/min

The PTA used was Hettiger with HP302 torch.

The PTA weld was applied on 1311 low carbon steel plate

measuring 50 x 200 x 20 mm (w x l x t).

The parameters used for the PTA tests were:

- Current: 125 A on the first layer

115 A on the second layer

- Welding speed 12 mm/s

- Feed rate 30 g/min

- Feed gas (Ar) 1 l/min

- Shield gas (95%Ar + 10%H) 12 l/min

- Plasma gas (Ar) 1,5 l/min

- Weld width 10 mm

Dilution was measured on the top layer using an XRF Niton

XL2 gun for the laser cladded coatings and EDS method for

the PTA ones.

Metallographic test samples were prepared by cutting the

samples perpendicular to the welding direction. These were

placed in a mounting press which was filled to 1/3 with a glass

fibre resin followed by 2/3 with Bakelite resin. The samples

were processed under high pressure during the heating and

cooling cycles. Finally the samples were plane ground and

polished with 9µm subsequently followed by 3µm and lastly

1µm DP (diamond polish) suspension. The etching agent used

was glyceregia which consists of 45 ml glycerol 98% (purum),

15 ml HNO3 and 30 ml HCl.

The samples were evaluated metallographically by LOM and

SEM. The hardness HV10 was measured and calculated as an

average of 7 measurements.

Results and Discussion

The pictures of the three laser coated sample together with the

microstructure can be seen in Fig. 1, 2 and 3.

Figure 1: 316 as laser coated on OD30/ID25 tube

ITSC 2015- Int. Thermal Spray Conf., May 11-14 2015, California, USA

Figure 2: 316 as laser coated on 10mm thick plate

Figure 3: 316 as laser coated on 20 mm thick plate

Each of the coatings in the previous figures show an overview

of the coatings microstructure.

The microstructure of the coatings consists of austenite and

delta ferrite. Fully austenitic stainless steels are known to be

prone to hot cracking during welding. In order to prevent the

occurrence of hot cracking, 4-10% ferrite should be present in

the finished weld. This can be achieved via laser cladding

processes due to the high cooling rate [1].

Figure 4 displays an even dendritic structure with coarser

dendrites, which have longer time to grow due to the slowest

cooling rate. This component was too small and overheated

during welding which lead to slower cooling rate compared to

the other components.

Figure 5 shows a mostly even structure, but finer due to faster

cooling rate.

In case of the largest mass component, Fig. 6, the structure

obtained was the most uneven compared to the smaller ones.

Figure 4. Dendritic structure on tube

Figure 5. Dendritic structure on the 10 mm thick plate

ITSC 2015- Int. Thermal Spray Conf., May 11-14 2015, California, USA

Figure 6. Dendritic structure on the 20 mm thick plate

More etched pictures of the different microstructures are

presented in Fig. 7, 8, and 9.

Figure 7. Microstructure of coating performed on tube

Figure 8. Microstructure of coating performed on 10 mm thick

plate

Figure 9. Microstructure of coating performed on 20 mm thick

plate

The microstructures of the coatings performed on the plates

were very similar (see Fig. 8 and 9). When the coating was

applied to the smallest component, the tube, a lower amount of

delta ferrite was obtained as a result of the lower cooling rate.

The outcome was a small hot crack in the structure which is

illustrated in Fig. 7.

The PTA and laser clad samples were compared on the 20 mm

thick plate. The microstructures are shown in Fig. 10

Figure 10. Microstructure of 316L as laser welded (top) and

PTA welded (bottom)

ITSC 2015- Int. Thermal Spray Conf., May 11-14 2015, California, USA

Figure 10 displays an altered size of the dendritic structure:

finer in the laser cladded samples due to the faster cooling rate.

Material 316L when laser welded shows excellent pitting

corrosion, which was presented in an earlier paper [2]. A finer

structure indicates better corrosion resistance compared to a

coarser one, [3] and [4]. This should be further investigated for

more clarification in the future.

The amount of delta ferrite in the PTA and laser cladded

samples has been investigated and the etched structures are

shown in Fig. 9.Murakami etching was used to reveal the delta

ferrite.

Figure 11. Microstructure of 316L as laser welded (top) and

PTA (bottom) showing the delta ferrite (etched in Murakami)

Figure 11 demonstrates a similar amount of delta ferrite in

both samples.

The WRC-1992 diagram [5] provides information on the

welding properties of the various types of microstructure as a

function of what alloying elements they contain. Chromium

equivalent is calculated using the weight percentage of ferrite

stabilizing elements and Nickel equivalent is calculated from

the weight percentage of austenite stabilizing elements. By

entering the Ni-equivalent over the Cr-equivalent for stainless

steel into a diagram according to WRC, one is able to find the

content of austenite and ferrite in the resulting microstructure.

The WRC diagram is today accepted as an improved version

of the Schaeffer or the De-Long diagram.

The composition of both coating has been calculated using the

EDS method for all the element beside C and N. These two

elements have been chemical analyses in the coating using the

IR analyze method.

Figure 12. Chemical composition of the laser welded (top)

and PTA welded sample (bottom)

After entering the range of the standard analysis and the actual

analysis, the Ni-equivalent and the Cr-equivalent are

calculated and shown in a diagram (see Fig. 13)

ITSC 2015- Int. Thermal Spray Conf., May 11-14 2015, California, USA

Figure 13. WRC-1992 diagram for the laser welded sample

(top) and PTA welded sample (bottom) showing the

percentage of delta ferrite.

The WRC diagram shows the same amount of delta ferrite in

both coatings at approximately 4%. Despite increased cooling

rate in the laser process, the differences in the chemical

compositions, lead to the same percentage of delta ferrite in

both cases.

As discussed earlier, dilution is greater in the PTA process due

to a higher amount of heat input during the process.

The dilution was also measured on the top layer deposition in

both cases and the results were:

4% for the laser cladded sample

9% for the PTA welded sample.

The dilution is also illustrated in Fig. 14, by using the line

analyse from the substrate through the coating.

Figure 14. Line analysis of the laser coating (top) and PTA

coating (bottom)

As it can be seen in Fig. 14, a steeper curve is obtained when

the laser process is used.

The hardness has been measured and a higher hardness was

obtained in the laser cladded sample, 166 HV10, compared to

the PTA laser welded sample, 150 HV10. The difference is

due to the different cooling rates between the two methods but

also as a result of composition transformation due to higher

dilution in the PTA sample.

The impurities in the substrate can influence the properties of

the final coating even in small quantities.

A 316L coating was applied on the low Carbon steel

component of two different qualities: one with 0,28%Cu and

another one containing 2% Cu.

The dilution was measured in both cases and following results

were obtained:

- Low C steel 0,28%Cu dilution 5%

- Low C steel 2%Cu dilution 11%

This proves that even minor variations in trace elements in the

substrate can result in different properties of the final coating.

Conclusions

Despite following identical setup procedure, the final

quality of the clad is dependent on several factors:

• Material used for cladding on component

• Component geometry

• Component size

• Size & number of impurities in the substrate

The geometry of the component can influence the

cladding pattern needed in order to get a good quality

clad

The final structure of the clad is a function of the

cooling rate, which is dependent on the component

size for the same process parameters

A finer structure is obtained when laser cladding

process is used compared to the PTA process. This

Si Cr

Ni Mn

Fe Mo

ITSC 2015- Int. Thermal Spray Conf., May 11-14 2015, California, USA

leads to higher hardness for the laser coating versus

the PTA coating.

Delta ferrite formed during the welding process is

critical in order to obtain a crack free structure

Dilution can be influenced by the chemistry of the

substrate.

Acknowledgments

The author would like to thank Mr. Kari Westerling and Mrs.

Barbara Maroli for their vital contribution to the successful

outcome of this work.

References

[1] ASM Speciality Handbook, Stainless Steels. ASM

International – The Materials Information Sociaty

[2] Kampanis, Nicholas and Hauer, Ingrid. “Propeller Shaft

Repair for a Large Ferry with the Aid of Laser Cladding”,

Surabaya, Indonesia : s.n., 2010. ICSOT Conference.

(Roussakis SA, Hellas, Höganäs AB, Sweden).

[3] Li, Zhen, “Influence of Cold Work and Grain Size on the

Pitting Corrosion Resistance of Ferritic Stainless Steel”,

Seattle : Minerals, Metals and Materials Society/AIME, 2010.

TMS 2010 Annual Meeting and Exhibition.

[4] Carvalho, Felipe Leal, “Influence of Austenitic Grain Size

on the Stress Corrosion Cracking Resistance of Steel Applied

in Sour Service Environments”, Brasil : 61st Annual ABM

International Congress, 2006.

[5] http://www.migweld.de/english/home.