Carbonitriding Effect on Fatigue Behaviour and Mechanical Properties of Steel Beam

This work deals with the influence of carbonitriding on the fatigue behavior of notched beam (v-notch) with a depth of 1mm and an angle of 45° is studded. The experimental work include mechanical tests, heat treatments and fatigue test. The heat treatment is done by using carbonitriding at a constant temperature of 800 °C, and soaking time variation 30, 60, and 90 minutes, then followed after treatment by quenching of water and tempering process. The experimental fatigue test results of this work affected by soaking time. The fatigue test has been performed on a cantilever rotating-bending samples. The result shows that the fatigue strength and lifetime increased with increasing soaking time. It is found that The Fatigue Life Improvement Factor (FLIF) at time 30, 60 and 90 minutes for notched beam (v-notch) was increasing by 11.3%, 18.6% and 20.2% respectively with respect to un-treat specimens.


Introduction
Fatigue is a condition in which Mechanical qualities of materials deterioration and one or more cracks slowly start to spread under the influence of dynamic load, which eventually leads to cracks [1].Where it was discovered that movement caused by fatigue accounts for 80-90% of service failures in metal components.Thus, there are three steps to fatigue failure: crack initiation, crack propagation, and fracture [2][3].Because they can provide extra features like high strength, thermal barrier, and corrosion, surface engineering techniques have attracted a lot of attention.The low carbon /mild steel is relatively soft, weak, low tensile strength, and easy to form.Therefore, it is significant to improve those properties by surface treatment, there are several ways to modify the fatigue behavior of low carbon steel like, laser, shot peening, induction hardening and carbonitriding…etc.Carbonitriding is method of surface heat treatment process.At temperatures between 850°C and 880°C, carbon and nitrogen diffuse into the surface of the component.The surface is first saturated with carbon and nitrogen before going through the hardening and tempering processes.The fatigue strength is improved by the hard layer of carbonitrides, nitrides, and carbides generated at the surface [4].[5], examined the surface toughness of carbonitrided LCS, MCS, LAS, and HAS high carbon alloy steels in various quenching media (air, oil, brine).At temperatures ranging from 790°C to 920°C, heat treatment procedures were carried out.To observe its impact on the hardness of the LCS surface, the heat treatment time was varied from 30 minutes to 120 minutes with a 30-minute break in between.Conclusion: The hardness of surface LCS rises with the duration of heat treatment, however a 90minute heat treatment generated the surface that was the toughest.[6], investigated the fatigue behavior of low-alloyed steel specimens.The impact of the hardening procedure with the subsequent grinding and ion nitriding process has been compared and contrasted.The experimental study has been carried out in two stages: residual stresses and cyclic bending fatigue testing.It has been shown that in the area of high-cycle fatigue, the ion nitriding process performs better than the quenching process followed by grinding in terms of fatigue life.Victor, [7] cyanide treatment (CN) at high temperatures (800-950°C) in a salt bath was studied to see how it affected the hardness of the material.For testing, low carbon steel was used.All samples had a maximum hardness of 63.5 HRC, which is an increase of 11.3% over untreated samples.The results show that when cyanide is applied to steel, the hardness increases to ideal levels if cyanide (CN) is processed at 850°C.Ghanem and Terres, [8] studied experimental tests of three points fatigue flexion , it is determined how the period of gas and carbon nitriding affects the improvement of the fatigue limits of low steel samples, To cause the tested steel to simultaneously absorb nitrogen and carbon, the specimens were carbonitrided at 870°C for seven and eight hours in an endothermic gas environment that contained a significant amount of the gas mixtures methanol and ammonia.Followed by tempered for one hour at 200°C and quenched in oil at a temperature of 60°C.Results indicated that fatigue resistance has improved.The aim of this work show effect of cyaniding salt bath on the fatigue life performance of low carbon steel beams (vnotched and un-notched) under reversed rotary bending stress at various time soaking, and also investigation the effect of carbonitriding on mechanical properties.

Impact Test
Impact testing was performed to determine the impact resistance after and before treat samples by carbonitrid by calculating the amount of energy absorbed during fracture.Impact test was achieved for the selected material for Izod method by using the impact-testing machine of type (Brooks AME 01-19) according to ISO A 370.

Hardness Test
Vickers method was used to measure, the hardness of a LCS during a length of time (10 sec) and under a force of (100 g).The hardness sample is as shown in Fig 2, where it shows the location of the hardness from the surface down to the core of the sample to measure the hardness of the metal after heat treatment by cyanide.

Surface Heat Treatments 2.2.1. Specimen Preparation
The design of fatigue specimens depending on the ASTM 606-80, by using machine CNC.Fatigue specimens are designed with dimensions that comply with instrument testing standards for cylindrical specimens as shown Fig. 3.The specimens divided into two groups, first group contain notched beam and second group un-notch beam.The notches were V shape with angle (α= 45  ) to a depth (h=1 mm) [10].

Carbonitriding (Cyanide Salt Bath)
Cyanide used to create a hard layer on the surface of low carbon steels.Both carbon and nitrogen are diffuse on surface metal during this process.The composition of Cyanide salt bath is 61% NACN, 24% KCL, and 15% K 2 CO 3 [8].Where specimens are putted inside the molten salt, which is heated to an internal temperature of 800°C, and are maintained there for intervals of 30, 60, and 90 minutes.The samples are then directly cooled in water.According to [11][12] tempering for 120 minutes at 180°C to reduce brittleness.Fig. 5 illustrating the cyanide process in terms of time and temperature.

Fatigue Test
The test is conducted using a rotating bending machine type (WP 140).This type with specimen features a cantilever beam with a totally reversed load and constant amplitude, as shown Fig 6 .A bending load with a 3000 RPM speed is applied to the cantilever-rotating sample's end.A series of tests was performed by representing a sample of a stress cycle that is heat treated with a molten salt bath, and the number of cycles until failure was determined.Data are drawn as stress with Number cycles to failure for each of the samples.It is essential to recognize that every S-N curve obtained in this study has a minimum of 7 samples.The bending moment values can be determine from equation [13].
Where: σ f is maximum alternating stress (MPa), F is applied load (N) L is bending arm = 106 ± 0.1 mm d is diameter of the specimen = 8 ± 0.1 mm.

Microscopic Examination
According to [9], cylindrical specimens underwent microscopic analysis of the surface layer using an Olympus BX60M high-resolution optical microscope.Treatment done by carbonitriding.The test piece's surface is ground using emery papers of various grades.Having polished the test samples using a rotary polishing machine and diamond paste until they have a surface that resembles a mirror, Nitric acid (2%), and alcohol (98%) are used to etch the polished surfaces.Then, to understand the microscopic structures produced by the diffusion process, the (G.K.B.,CCD) camera was attached to the microscope and photographed.Fig. 7 display the material's microstructure in the received state.according to [14], this caused the metal to become extremely hard.Notes that layer white (compound layer) increase with increasing time soaking at constant temperature 800°C.This layer had high hardness.

Hardness Test
Fig. 9 show the change in hardness values.The hardness decreases towards the center of the sample due to the diffusion of carbon and nitrogen from the surface to the center of the sample.The hardness values are taken from surface to the center at distance 0.05 mm for each reading.The depth of hardening layers at a temperature ( 800°C ) are (0.33 mm, 0.59 mm and 0.84 mm) within soaking time of (30, 60, 90) minutes respectively.The hardness of low carbon steel increases with the increasing soaking time of carbonitriding according to [8].
Figure 9.The relationship between hardness of specimen's carbonitrid and the distance from surface to core.  3. Presented the results of bending fatigue testing at various levels of carbonitriding (C.N), where the specimen life is calculated using the mean of three repeated tests.Fig 10 .Show the S-N curves of carbonitriding (C.N).This figure shows the fatigue behavior after surface thermal treatments and the effect of soaking time.After surface hardening via carbonitriding, there is a discernible difference in fatigue behavior.Due to the formation of a high hardness surface layer, this layer resulted from diffusions of carbon and nitrogen atoms together inside the metal surface due to the difference in concentrations from of high pressure towards low pressure.As a result of this diffusion iron nitride and carbide (Fe 4 N, Fe 3 C) is formed [9], leads to an improvement in the fatigue performance of each hardening process.Where the surface of the steel becomes smoother and has a higher hardness, which leads to delay crack initiation and growth it.The surface hardness has provided an improvement in the fatigue strength because the hard layer prevents plastic flow [7][8][9].Table 3. Experiment fatigue stresses with life of specimens.

Life of Specimens (Cycles) Time(h) Life/1h
Life   4. Represent the experimental fatigue strength (endurance limit) and the experimental S-N equations (Basquin's equations) carbonitriding (cyaniding).The Basquin's equation is a power law regression and can be given by equation 3 [15].
a and b curve fitting constant.Observed that the fatigue strength of the selected material increase with increasing soaking time, due to the spread of nitrogen and carbon atoms formed hard layer with different depth.In addition to formed iron carbide and iron nitride.Fatigue strength improvement factor (FSIF) can be determined using the equation below.
X 100 % (4) Where, σe is the fatigue strength at 10 6 cycles for surface hardened steel, σe ref. is the fatigue strength at 10 6 cycles of reference material which it un-treated specimen .Fig 11.Show the effect of carbonitid on FSIF% at different soaking time.It was concluded that the FSIF% increase with an increase time for cyaniding process about 31.26%,52.36%and 61.87% with time 30 , 60 and 90 minutes.The Fatigue Life Improvement Factor (FLIF) can be determined using the equation below

Un Notch Specimens
The S-N equation (Basquin equation) which is a power law regression, as stated in equation ( 3) for plain samples is found experimentally and presented in tables 6. Plain Low carbon steel has been treated by heat treatment of carbonitriding at time 0.5 hour.Table 6.Experimental S-N curve equation and fatigue strength σe at 10 6 cycles function for carbonitriding plain carbon steel.from surface to core about 0.33mm as a result diffusion carbon and nitrogen from salt bath.Variation in hardness between surface and core lead to delay cracks and improved fatigue behaviors.

Conclusion
From the results of this study, the following points can be concluded:  Fatigue life time increased compared to untreated metal. Fatigue life improvement factor enhancement for notch specimens treated about 20% . Fatigue strength increased after treatment about 281, 327 and 347.594MPa for specimen's notch.
After treatments, fatigue limit and fatigue resistance enhanced. Tensile test increased due to treatment by carbonitred .wherethat Yield Strength 991MPa and ultimate strength 1390 MPa Recommendation  Studding the effect of carbonitriding at constant time and a variable temperature on the fatigue life behavior of V-notch specimens. Studding the effect of carbonitriding on different shape of notch on fatigue life behavior such as U-notch and compare the result with V-notch and plain specimen. Studding the effect of carbonitriding followed by Nd: YAG laser with different powers on fatigue life behavior U-notch specimens made of low carbon steel. Studding the effect of carbonitriding on fatigue life behavior of V-notch specimens made of medium carbon steel.

Figure 1 .
Figure 1.A schematic diagram of a tensile test sample according to (ASTM-A 370) specifications.

Figure 4 .
Figure 4. Dimensions of V shape notch specimens.
Fig 8 shows the microstructure of steel after surface treatment by carbonitride.It is noticed by examining the structure that a white layer (the composite layer) appears due to the diffusion of nitrogen and carbon together into the metal surface.The chemical reaction between the alloying elements of steel and the carbon and nitrogen atoms from heat treatment results in the formation of (Fe 3 C) and (Fe 4 N)

Figure 7 .
Figure 7. Microstructure of material as received.

Figure 13 .
Figure 13.Effect carbonitriding (S.N) curve on plain carbon steel at time 30 min.Observed from Fig 13.Improvement in fatigue life where fatigue strength increased about to 354.75MPa compared with untreated 256.2 MPa.Due to soft surface specimen and formed hard layer

Table 1 .
In this test, Izod method was used.Table1.Shows the results of the impact test at room temperature, for material untreated and treated, three specimens was tested for each case.Observed in this table increase in amount of energy absorbed after treatment due to diffusion nitrogen and carbon with different depth.This improvement in energy absorbed due to of layer hardness, iron nitride and carbide formed on surface specimen, while the core remain soft.Impact test result.Table2show result of tensile test after and before treatment by cyanide salt bath an increase of yield strength and ultimate strength with increasing time soaking of salt bath compared to untreated.Observed maximum yield strength and ultimate strength 991 MPa and 1390 MPa respectively.

Table 5 .
Percentage of Fatigue Strength Improvement Factor (FSIF %) of carbonitriding at different time.
Figure 11.Fatigue Strength Improvement Factor for different time for carbonitid.