## GH4169合金圆盘时效过程残余应力的演化规律研究

1. 钢铁研究总院高温合金新材料北京市重点实验室 北京 100081

2. 北京钢研高纳科技股份有限公司　北京　100081

3. Department of Engineering, University of Leicester, Leicester, LE1 7RH, UK

4. ISIS Neutron Source, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, OX11 0QX, UK

## Study on the Evolution of Residual Stress During Ageing Treatment in a GH4169 Alloy Disk

QIN Hailong, ZHANG Ruiyao, BI Zhongnan,, Tung Lik Lee, DONG Hongbiao, DU Jinhui, ZHANG Ji

1. Beijing Key Laboratory of Advanced High Temperature Materials, Central Iron and Steel Research Institute, Beijing 100081, China

2. CISRI-GAONA Co. , Ltd. , Beijing 100081, China

3. Department of Engineering, University of Leicester, Leicester, LE1 7RH, UK

4. ISIS Neutron Source, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, OX11 0QX, UK

 基金资助: 国家重点研发计划项目.  (No.2017YFB0702901)国家自然科学基金项目.  (No.U1708253)

Corresponding authors: BI Zhongnan, senior engineer, Tel:(010)62188576, E-mail:bizhongnan21@aliyun.com

Received: 2018-09-07   Revised: 2019-04-08   Online: 2019-07-24

 Fund supported: Supported by National Key Research and Development Program of China .  (No.2017YFB0702901)National Natural Science Foundation of China .  (No.U1708253)

Abstract

GH4169 alloy, a precipitation-strengthened nickel-iron base superalloy, has been widely used in aerospace and energy industries due to its excellent high-temperature strength which derived from the coherent phases (γ″ and γ'). To form these precipitates, the manufacturing process of GH4169 usually involves solid solution heat treatment followed by rapid cooling and double ageing heat treatment. Significant residual stresses are induced during rapid cooling and then partially relieved during the subsequent ageing treatment. However, the reduced residual stress after ageing are still large enough to affect the final machining operations, resulting in the component exceeding the dimensional tolerances if they are not well considered. Furthermore, residual stresses in the final components may lead to further distortion beyond estimation during service, which could deteriorate the engine performances. In the present study, the evolution of residual stresses at heating, isothermal ageing, and air-cooling stages of ageing heat treatment in a GH4169 alloy disk was characterized by in situ neutron diffraction. Considering the effect of residual stresses on the precipitation behavior of γ″, two different types of stress-free samples were used as the basis for the stress analysis. The results show that significant residual stresses were induced during water quenching, which were found to be 340.62 MPa tensile in hoop/radial directions and 33.34 MPa compressive in axial direction in the center of the disk. Subsequently, an in situ ageing heat treatment was undertaken at 720 ℃ for 8 h. During the heating stage, the yield strength of the material decreases with increasing temperature, leading to residual stress relaxation through plastic deformation from 340.62 MPa to 227.67 MPa in hoop/radial direction in the disk center. At the isothermal ageing stage, residual stresses relieved apparently by about 40 MPa during the first 100 min, later on a slower linear relaxation remained for the rest of the ageing heat treatment. The strength of the alloy increased and the creep rate decreased due to the formation of γ″ and γ′ strengthening phases, indicating that most of stress relaxation occurred as a result of creep deformation at the early stage of isothermal ageing. The magnitude of residual stress was almost invariable in the subsequent air-cooling stage.

Keywords： superalloy ; ageing treatment ; residual stress ; in situ neutron diffraction

Hailong QIN, Ruiyao ZHANG, Zhongnan BI, Lee Tung Lik, Hongbiao DONG, Jinhui DU, Ji ZHANG. Study on the Evolution of Residual Stress During Ageing Treatment in a GH4169 Alloy Disk. Acta Metallurgica Sinica[J], 2019, 55(8): 997-1007 doi:10.11900/0412.1961.2018.00428

GH4169合金(国外牌号Inconel 718)是目前应用最为广泛的沉淀强化型镍基变形高温合金，在650 ℃以下具有较高的强度和塑性及良好的抗疲劳和耐腐蚀性[1,2]，是航空、航天、石油化工及核能等领域大量应用的关键材料[3,4,5]。GH4169合金以γ″-Ni3Nb相为主要强化相，γ′-Ni3(Al, Ti)相为辅助强化相。标准热处理后的显微组织主要由基体γ相、弥散分布的γ″和γ′、δ相以及少量MC相组成。为了满足预期的强化水平，固溶后的工件常进行快速冷却(水淬或油淬)，以避免主要强化相γ″在冷却时析出并发生粗化[6]。与此同时，由热应力造成的不均匀塑性变形则会产生约400 MPa的残余应力[7,8,9]。Dye等[7]和Rist等[8]采用中子衍射法和有限元模拟相结合的方式，研究了淬火过程残余应力的产生过程及分布状态，认为在快速冷却过程中避免了γ″相的析出，淬火过程中的组织变化可以忽略，同时，由热应力产生的不均匀塑性变形导致了较大残余应力的产生，其分布状态为“外压内拉”。

## 1 实验方法

### 图1

Fig.1   In situ neutron diffraction experiment during ageing treatment

(a) heating by ceramic blanket under clad insulation (b) covering by heat-preservation cotton

GH4169合金圆盘在自主设计的原位加热装置上进行实验(图1a)。加热毯是由块状Al2O3中包裹NiCr丝组成。根据圆盘的几何尺寸，加热元件被设计成覆盖在样品表面。在测试点附近摘除相对应的陶瓷片及加热丝，以方便中子束流的通过。圆盘样品的端面中心和边缘分别用电阻点焊的方式连接上K型热电偶，以监控加热时的温度曲线。在加热毯的外面和上面包裹保温棉和少量Al箔来进行保温。研究发现外部材料(包括保温棉和Al箔)对中子穿透性能的影响可以忽略不计[10]

### 图2

Fig.2   Schematic of neutron path and location for neutron diffraction

### 图3

Fig.3   Schematic of stress-free s0 sample

### 1.4 残余应力及其误差的计算

$ε=Δa/a0=(t-t0)/t0$

$σ11=E(1+v)ε11+vE(1+v)(1-2v)(ε22+ε33)$

σ22σ33的计算公式可依次类推。式中，E为弹性模量，v为Poisson比。不同温度下的Ev数据均取自文献[26]。ε11ε22ε33为正应变。残余应力拟合分析误差主要来源于2部分：(1) 由Pawley拟合所造成工件测试部位的统计误差$uai$，(2) 无应力标样的统计误差$ua0$。因此，应变误差($uεi$)可以通过下式计算得出：

$uεi2=uai2⋅a02+ua02⋅ai2a04$

## 2 实验结果

### 图4

Fig.4   Temperature profile of the disk during in situ heating experiment

### 2.2 组织变化

GH4169合金的标准热处理制度包括固溶和时效两步热处理。Geng等[6]的研究结果表明，当固溶后平均冷却速率大于20 ℃/min (980~700 ℃区间)时，可以避免γ″和γ′相在冷却过程中析出。图5所示为固溶水淬后圆盘试样的显微组织，平均晶粒尺寸为35 μm (图5a)，晶界处有δ相分布，晶内为单一奥氏体，无强化相(γ″和γ′)在晶内析出(图5b)。固溶冷却后的显微组织主要由基体γ相以及晶界处的少量δ相组成。在随后的时效升温阶段，温度低于650 ℃时，第二相析出动力学非常缓慢[27]。因此，在淬火和时效升温过程中，圆盘工件内的组织变化较小，可以忽略不计。

### 图5

Fig.5   OM (a) and SEM (b) images of the microstructures of disk after quenching

### 图6

Fig.6   TEM bright-field images of s0-1 (a, b), s0-2 (c) and s0-5 (d) samples aged for 0.5 h (a, c) and 8 h (b, d)

### 2.3 残余应力的演化行为

Table 1  Evolution of lattice parameter (a), strain and corresponding residual stress during the heating process

Temperature

Direction

a / nmStrain / 10-6Stress / MPa
ValueErrorValueErrorValueError

20

Axial0.3598960.0000072-1168.4219.98-33.349.88

340

Axial0.3614860.0000079-1292.9821.83-60.709.04

530

Axial0.3625500.0000085-1279.6922.59-65.718.99

720

Axial0.3635020.0000088-1351.9624.18-77.749.94

$y=y1+A1exp(-tl)+A2exp(-tm)+A3exp(-tn)$

### 图7

Fig.7   Profiles of a in the center of disk during isothermal ageing process

### 图8

Fig.8   Evolution of residual stress in the center of disk during isothermal ageing process on the basis of dynamic non-stress standard sample (a) and static non-stress standard sample (b)

### 图9

Fig.9   Evolution of residual stress in the center of disk during ageing treatment

## 3 分析讨论

### 图10

Fig.10   Neutron diffraction spectra at the beginning (5 min) and the end (480 min) of creep test in longitudinal direction (a), and the separation of overlapped peak (200) (b)

### 图11

Fig.11   Evolution of stress fitting error with increasing ageing time

### 3.3 升温阶段残余应力的释放机制

$σe=(σ1-σ2)2+(σ2-σ3)2+(σ3-σ1)22$

### 图12

Fig.12   Yield strength of as-quenched materials vs temperature

### 图13

Fig.13   Evolution of microstructure and mechanical properties of GH4169 alloy during isothermal aging treatment

(a) volume fraction of γ″ and γ

(b) average diameter of γ″ and γ

(c) yield strength at room temperature

(d) creep strain of as-quenched GH4169 alloy at 720 ℃

## 4 结论

(1) 利用原位中子衍射法分别表征了时效热处理中升温、保温和空冷3个阶段残余应力的演化行为。考虑到工件内部残余应力对γ″相析出行为的影响，在分析应力的过程中采用了2种不同形式的无应力标样作为基准。静态无应力标样取自时效不同时间后的平行圆盘试样，与原位衍射圆盘试样中心处的组织状态高度一致，以其为基准分析应力时，结果较为准确。

(2) 时效升温阶段中材料强度逐渐降低，部分残余应力会通过塑性变形的方式进行释放，圆盘中心处旋向/径向残余拉应力的数值从淬火后的340.62 MPa降至227.67 MPa，释放量占33%。

(3) 时效保温阶段中，残余应力主要通过蠕变变形的方式进行释放，释放量约占15%。伴随着γ″强化相在晶内逐渐析出，材料强度随之提高，蠕变抗力增大，因此保温过程中残余应力松弛主要集中在时效早期。

(4) 由于时效后空冷产生的热应力较低，而此时材料屈服强度较高，难以发生塑性变形，所以残余应力与空冷前相比保持同一水平。

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