This document summarizes a study that evaluated the abrasive wear resistance of coatings deposited on 420 stainless steel substrates using the Twin Wire Arc Spray technique. The coatings were tested under dry and wet abrasive conditions according to ASTM standards using two different loads. The as-sprayed coatings and those subjected to tempering and cryogenic heat treatments were analyzed. Microstructural characterization found porosity of around 3.26% in as-sprayed coatings. Cryogenic treatment increased microhardness while tempering improved dry and wet abrasion resistance. Main wear mechanisms identified were splat delamination, plastic deformation, micro-cutting and fracture. The most severe wear in wet tests occurred on coating broadsides while dry

1. ABRASIVE WEAR IN THERMAL SPRAY COATNGS WITH THE TWAS TECHNIQUE
USING 420 STAINLESS STEEL
DIP-CUCEI, Maestría en Ciencia de Materiales, Universidad de Guadalajara, A.P 307, Zapopan Jal. CP45100
H. Monjardín, E. Rodríguez, O. Jiménez, M. Flores, E. Mojardín.
Project in colaboration with SURESA Company, Carretera libre a Zapotlanejo
No. 2838 Col. Jauja C.P. 45426 Tonalá, Jalisco.
SURESA
Spray coatings for your needs
Contact: hector.monjardin@hotmail.com
ABSTRACT
One of the most profitable thermal spray techniques is the Twin Wire Arc Spray (TWAS) due to its relatively good cost-benefit compared to other thermal spray techniques. Coatings of AISI 420 mar-
tensitic stainless steel was deposited using the TWAS technique. The main goal of the present study is the evaluation of the abrasive wear resistance in dry and wet conditions according to the ASTM
G-65 and G-105 standards respectively, using two different loads 30 lb. and 50 lb. The same wheel rpm and number of revolutions for both conditions were used in order to compare the differences
in mass loss, and the wear mechanisms in the coatings. The abrasive wear resistance of both, the As-sprayed condition and heat treated samples, such as Tempering (205o
C) and Cryogenic (-196o
C,
2.5o
C/min cooling rate) was studied to compare the effect of such treatments on the 420 SS TWAS coatings. Results of the microstructural analysis by optical, electron microscopy, and by EDS are
shown. Also, area percentage porosity according to ASTM E-2109, HV50 Hardness profiles in the different heat treatments, and the mass lost curves due to the dry and wet abrasion for each experi-
mental condition are presented. Images of the main abrasion mechanisms were obtained by SEM.
EXPERIMENTAL DETAILS
A) Coating process
Table II Wire arc spraying process parameters
Parameter Value
TWAS Machine Metco 4RC
Adhesion coat wire 1/16in Ni-AL
Wire 1/16 in. 420 stainless steel
Voltage 32 V
Amperage 200 A
Gas pressure 0.6 Mpa PG and 0.4 Mpa SG
Gas Air
Nozzle-Substrate distance 150mm
Projection angle 90°
Wire feed Automatic
Final coating thickness >2mm
IV)
Fig. 1 I) and II) TWAS process using a Fanuc Robot mod. S420F, III) 304 SS substrate plate coating with 420 SS, IV) TWAS Machine
Metco 4RC,
B) Specimens preparation sequence
The specimens were final finishing until 2000 grade emery paper. Surface Roughness Tester mod. SRT-6210
was used. The average roughness of the specimens was Ra =0.94µm.
A) Final Roughness
RESULTS
B) Coating structure C) Coating HV microhardness
C) Wet and Dry abrasion machine
Table III Wet and Dry parameters
“Wet Wheel abrasion” parameters
Slurry
.940 kg De-ionized water and 1.5 kg 40-60
mesh silica sand
Wheel velocity 120.5 RPM
Number of wheel revolutions 1000 Rev.
Loads 30 lb. and 50 lb.
“Dry Wheel abrasion” parameters
Sand 40-60 mesh silica sand
Wheel velocity 120.5 RPM
Number of wheel revolutions 1000 Rev.
Sand flow 190 gr/min.
Loads 30 lb. and 50 lb.
Fig. 9 420 SS TWAS coating structure. Porosity around 3.26%
was obtained using the ASTM E-2109 standard.
Fig. 11 420 SS TWAS coating (As sprayed+ Cryo-
genics) HV mark, 807.59 HV.
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
HVmicrohardness
Vickers microhardness
As
As+TT
As+Crio
As+TT+Crio
As+Crio+TT
As+TT+Crio+TT
Fig. 12 Vickers microhardness for all experimental
conditions using a Future Tech FM-800 microdurome-
ter, 50 gr load and 15 sec.
D) Wet and dry Abrasive wear
Fig. 8 Sand Wheel abrasion machine ASTM G-
65 modified to ASTM G-105 Wet abrasion.
Fig. 13 I) Wet wear abrasion mark in a As-sprayed condition 50 lb.
and II) 3D map of the wear mark using Vecco Dektak profilometer.
Fig. 14 I) Dry wear abrasion mark in a As-sprayed + Cryogenics con-
dition 50 lb. and II) 3D map of the wear mark using Vecco Dektak
E) Mass loss in dry and wet environments
I)
II)
III)
F) Wear mechanisms
Fig. 16 Low magnification SEM view of As-
sprayed+ Cryogenics , dry condition and 50 lb.
load, A) Deformed splat before remotion, B) Crater
formation due remotion.
Fig. 17 SEM view of As-sprayed+ TT 205o
C,
wet condition and 30 lb. load.
Fig. 18 SEM view of As-sprayed+ TT 205o
C,
Table I 420 stainless steel wire che-
mical composition
Element Wt (%)
Carbon 0.28
Manganese 0.42
Silicon 0.37
Molybdenum 0.15
Sulfur 0.03
Chrome 13.13
Iron 85.62
Source: Washington Alloy Datasheet
Fig. 2 Specimens sizing
Fig. 3 Cutting machine Metkon Met-
acut-M 250
Fig. 4 420 Thermal spray
coating specimens
Fig. 5 Heat treatments used,
Tempering and Cryogenics.
Fig. 6 Specimen grinding using
different flap sizes
Fig. 7 Specimen with a final fin-
ishing (2000 emery paper)
Fig. 10 EDS analysis of structure composition in As-sprayed+ Cryogenics
specimen condition.
Fig. 15 Mass loss graphs for dry and wet conditions.
CONCLUSIONS
I) An increase in microhardness occurs due the Cryogenic treatment.
II) There is a wear mark difference between the dry and wet abrasive environment, the most severe wear in the wet abrasion was local-
ized at the broadsides. While in the dry environment the wear is concentrated over all the wear mark.
III) The cryogenic treatment shown no improvement in the dry abrasion environment. While the specimens with a 205o
C tempering ex-
hibited an improvement for both loads 30 lb. and 50 lb. So, the coating hardness was irrelevant.
IV) The wet abrasion behavior had to much data dispersion. H13 steel reference specimens shown that the dispersion is due to the
coating characteristics, and not for wet abrasion machine.
V) The two-body abrasion mode are present and the main mechanisms in both conditions, wet and dry, were: the splats delamination,
plastic deformation (micro-ploughing), micro-cutting and fracture.
I)
153 um
216 um
II)
I)
II)
223 um
368 um
H13 Steel specimens