3.1 Correlation between parameters of surface strengthening (surface hardness, hardening zone depth) and the fatigue properties of different titanium alloys under CTT

To determine the correlation between parameters of surface hardening and fatigue properties of titanium alloys of various structural classes under conditions of thermodiffusion saturation in controlled gas medium in the surface layers of metal, (a) the hardened layers of different depth with identical level of surface hardening, K = const, l = var., and (b) the hardened layers of identical depth with different level of surface hardening, K = var., l = const, were formed. It allows to reveal the influence of level and depth of surface hardening and also determine the optimal combination of these parameters.

#### 3.1.1 α-Alloy VT1-0

## 3.1.1.1 Regulated surface hardening of VT1-0 titanium alloy under conditions of thermodiffusion saturation in controlled gaseous medium

To form the hardened layers with various ratios K and l, the determined relations considering the influence of temperature-time and gas-dynamical parameters of rarefied gas medium containing oxygen on the parameters of hardened layers and peculiarities of solid solution surface hardening of titanium alloys of various structural classes were used [7–12]. The used approaches for selection of parameters of thermodiffusion saturation under specified K and l are demonstrated in Figure 14 for alloy VT1-0 as an example.

The parameters of thermodiffusion saturation regimes of titanium alloy VT1-0

Example of a choice of VT1-0 titanium alloy thermodiffusion saturation's parameters: (1) <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa,

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium

The surface-hardened layers influence sufficiently the metal fatigue resistance. The results of fatigue tests of samples of alloy VT1-0 after regulated surface hardening by thermodiffusion saturation in gas medium are presented in Figures 16–19 and Tables 22 and 23. The obtained results allow analyzing the influence of the level of surface hardening and depth of hardened zone on the metal fatigue

in controlled gas medium containing oxygen which are selected based on the approach mentioned above which are refined experimentally and correspond to normative documents [13] are presented in Table 21. Parameters of surfacehardened layer and hardness distribution through the cross section of samples are

3.1.1.2 Influence of CТT on fatigue properties of α-titanium alloy VT1-0

Influence of inleakage rate (Iin) on VT1-0 titanium alloy surface hardness gain.

determined by means of durometry.

(2) <sup>P</sup> = 1.33 <sup>10</sup><sup>2</sup> Pa, and (3) <sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa.

DOI: http://dx.doi.org/10.5772/intechopen.82545

resistance.

93

Figure 14.

Figure 15.

Thus, the hardened layers of different depths with identical level of surface hardening can be obtained by means of changing the oxygen partial pressure and duration of saturation. The change of temperature influences the intensity of processes of thermodiffusion saturation. The influence of rate of oxygen leaking into the reaction chamber corresponds to the changing of partial pressure of chemically active component of gas medium (Figure 15). In addition the peculiarities of thermodiffusion saturation of titanium alloys of various structural classes should be taken into account. Thus, the hardened (gas saturated) layers with predetermined ratios K and l can be obtained due to changing of four parameters of thermodiffusion saturation (T, τ, PO2, and Il—rate of oxygen in leakage into the reaction camera).

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium DOI: http://dx.doi.org/10.5772/intechopen.82545

#### Figure 14.

3. Correlation between parameters of surface strengthening layers of

Nomograms for determination of parameters of CTT of titanium alloys (a) VT1-0 and (b) VT5 (the curves

hardness, hardening zone depth) and the fatigue properties of different

To determine the correlation between parameters of surface hardening and fatigue properties of titanium alloys of various structural classes under conditions of thermodiffusion saturation in controlled gas medium in the surface layers of metal, (a) the hardened layers of different depth with identical level of surface hardening, K = const, l = var., and (b) the hardened layers of identical depth with different level of surface hardening, K = var., l = const, were formed. It allows to reveal the influence of level and depth of surface hardening and also determine the optimal

different titanium alloys and their fatigue properties

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

titanium alloys under CTT

correspond to the level of surface hardening K = 25%).

combination of these parameters.

in Figure 14 for alloy VT1-0 as an example.

3.1.1 α-Alloy VT1-0

Figure 13.

reaction camera).

92

3.1 Correlation between parameters of surface strengthening (surface

3.1.1.1 Regulated surface hardening of VT1-0 titanium alloy under conditions of

structural classes were used [7–12]. The used approaches for selection of

ratios K and l can be obtained due to changing of four parameters of

To form the hardened layers with various ratios K and l, the determined relations considering the influence of temperature-time and gas-dynamical parameters of rarefied gas medium containing oxygen on the parameters of hardened layers and peculiarities of solid solution surface hardening of titanium alloys of various

parameters of thermodiffusion saturation under specified K and l are demonstrated

Thus, the hardened layers of different depths with identical level of surface hardening can be obtained by means of changing the oxygen partial pressure and duration of saturation. The change of temperature influences the intensity of processes of thermodiffusion saturation. The influence of rate of oxygen leaking into the reaction chamber corresponds to the changing of partial pressure of chemically active component of gas medium (Figure 15). In addition the peculiarities of thermodiffusion saturation of titanium alloys of various structural classes should be taken into account. Thus, the hardened (gas saturated) layers with predetermined

thermodiffusion saturation (T, τ, PO2, and Il—rate of oxygen in leakage into the

thermodiffusion saturation in controlled gaseous medium

Example of a choice of VT1-0 titanium alloy thermodiffusion saturation's parameters: (1) <sup>P</sup> = 6.6 <sup>10</sup><sup>2</sup> Pa, (2) <sup>P</sup> = 1.33 <sup>10</sup><sup>2</sup> Pa, and (3) <sup>P</sup> = 6.6 <sup>10</sup><sup>3</sup> Pa.

Figure 15. Influence of inleakage rate (Iin) on VT1-0 titanium alloy surface hardness gain.

The parameters of thermodiffusion saturation regimes of titanium alloy VT1-0 in controlled gas medium containing oxygen which are selected based on the approach mentioned above which are refined experimentally and correspond to normative documents [13] are presented in Table 21. Parameters of surfacehardened layer and hardness distribution through the cross section of samples are determined by means of durometry.

#### 3.1.1.2 Influence of CТT on fatigue properties of α-titanium alloy VT1-0

The surface-hardened layers influence sufficiently the metal fatigue resistance. The results of fatigue tests of samples of alloy VT1-0 after regulated surface hardening by thermodiffusion saturation in gas medium are presented in Figures 16–19 and Tables 22 and 23. The obtained results allow analyzing the influence of the level of surface hardening and depth of hardened zone on the metal fatigue resistance.

Figure 16.

Figure 17.

Figure 18.

95

K = 5%; l = 5 μm, (2) l = 30 μm, (3) l = 70 μm.

Fatigue curves of titanium alloy VT1-0, under rotating bending conditions, depending on the level of surface hardening when depth of hardened zone (gas saturated) is constant: (1) initial state K = 5%; l = 5 μm; (2) K = 25%; l = 30 μm (3) K = 50%; l = 30 μm (4) K = 70%; l = 30 μm (5) K = 90%; l = 30 μm.

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium

DOI: http://dx.doi.org/10.5772/intechopen.82545

Fatigue strength of titanium alloy VT1-0, under rotating bending conditions, as a function of level of surface

Fatigue curves of titanium alloy VT1-0, under rotating bending conditions, depending on depth of hardened (gas saturated) zone, when level of surface hardening is constant (a, K = 50%; b, K = 70%): (1) initial state

hardening when depth of hardened zone (gas saturated) is constant l = 30 μm.

#### Table 21.

Parameters of VT1-0 titanium alloy surface-hardened layers and thermodiffusion saturation's regimes.

#### 3.1.1.3 Influence of level of surface hardening

Fatigue strength σ<sup>1</sup> of titanium alloy VT1-0 is being increased initially (Table 2 and Figure 17) and then decreased with the increasing of level of surface hardening K from 5 to 90%, at constant depth of hardened zone (l = 30–35 μm). In other

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium DOI: http://dx.doi.org/10.5772/intechopen.82545

#### Figure 16.

Fatigue curves of titanium alloy VT1-0, under rotating bending conditions, depending on the level of surface hardening when depth of hardened zone (gas saturated) is constant: (1) initial state K = 5%; l = 5 μm; (2) K = 25%; l = 30 μm (3) K = 50%; l = 30 μm (4) K = 70%; l = 30 μm (5) K = 90%; l = 30 μm.

Figure 17.

Fatigue strength of titanium alloy VT1-0, under rotating bending conditions, as a function of level of surface hardening when depth of hardened zone (gas saturated) is constant l = 30 μm.

#### Figure 18.

Fatigue curves of titanium alloy VT1-0, under rotating bending conditions, depending on depth of hardened (gas saturated) zone, when level of surface hardening is constant (a, K = 50%; b, K = 70%): (1) initial state K = 5%; l = 5 μm, (2) l = 30 μm, (3) l = 70 μm.

3.1.1.3 Influence of level of surface hardening

Table 21.

94

Fatigue strength σ<sup>1</sup> of titanium alloy VT1-0 is being increased initially (Table 2 and Figure 17) and then decreased with the increasing of level of surface hardening K from 5 to 90%, at constant depth of hardened zone (l = 30–35 μm). In other

Parameters of VT1-0 titanium alloy surface-hardened layers and thermodiffusion saturation's regimes.

Titanium Alloys - Novel Aspects of Their Manufacturing and Processing

therefore increasing of fatigue properties is possible up to a certain level of harden-

Relative gain of fatigue strength is being decreased with increasing of depth of hardened zone l under constant level of surface hardening K (Figures 18 and 19, Table 3). The higher level of K, the rather fatigue strength of alloy is being

Solid solution hardening of surface layer of titanium alloys under conditions of thermodiffusion saturation by interstitial impurity (oxygen) can lead to the embitterment of hardened in this way layer and its brittle failure under conditions of the repeated loading. Because of this the investigations of fracture of samples were carried out after fatigue tests by rotating bending. Results of fractographical investigations of near-surface parts of fractures of titanium alloy VT1-0 samples with different levels of surface hardening after fatigue tests by rotating bending are

Fractograms of near-surface part of fractures of titanium alloy VT1-0 samples with different levels of surface hardening after fatigue tests by rotating bending: (a) K = 70%, l = 30 μm, σ<sup>1</sup> = 310 MPa; (b) K = 50%,

l = 70 μm, σ<sup>1</sup> = 285 MPa; (c), (d), (e) K = 70%, l = 70 μm, σ<sup>1</sup> = 260 MPa.

ing higher of which fatigue properties is being decreased.

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium

decreased with the rising of depth of hardened zone (Figure 19).

3.1.1.4 Influence of depth of hardened zone

DOI: http://dx.doi.org/10.5772/intechopen.82545

presented in Figure 20.

Figure 20.

97

#### Figure 19.

Fatigue strength of titanium alloy VT1-0, under rotating bending conditions, as a function of depth of hardened zone, when level of surface hardening K is constant: (1) K = 50% (2) K = 70%.


#### Table 22.

Fatigue strength of alloy VT1-0, under rotating bending conditions, depending on level of surface hardening K, when depth of hardened zone is l is constant.


#### Table 23.

Fatigue strength of titanium alloy VT1-0, under rotating bending conditions, depending on depth of hardened zone l, when level of surface hardening K is constant.

words, fatigue strength has a maximum. Relative gain of fatigue strength Δσ<sup>1</sup> is the highest when K = 70%. Such character of changing σ<sup>1</sup> can be explained by the improvement of fatigue properties due to dissolution of oxygen in metal with formation of solid solution that is accompanied by the appearing of compressing stress. On the other hand, metal is embrittled due to solid solution hardening by oxygen dissolution. One or another factor dominates the defined conditions,

Surface Treatment of Titanium Alloys in Oxygen-Containing Gaseous Medium DOI: http://dx.doi.org/10.5772/intechopen.82545

therefore increasing of fatigue properties is possible up to a certain level of hardening higher of which fatigue properties is being decreased.
