航空機 및 舶用機關 部品用 異種 耐蝕·耐熱鋼의 摩擦壓接과 壓接部의 疲勞强度에 關한 硏究 = Studies on Friction Welding and Welded Joint's Fatigue Strength of Heat Resisting Dissimilar Steels used for Aeroplane or Marine Engine Components

국문 초록 (Abstract)

最近 耐蝕, 耐熱鋼의 利容度가 增大되었고, 一部 特殊 耐熱綱이 國産化 되기 이르렀으며, 自動車, 船舶, 航空機 等 産業機械工業의 發達에 따라 生産面에서도 工程의 短縮, 生産原價의 切減, 疲勞設計와 安全設計, 特殊 異種材料의 完全한 接合性 必要 等으로 因해 앞으로 耐熱綱에 對한 摩擦壓接의 利用이 不可避하게 될 것을 勘案하여, 850。C 以下의 耐酸, 耐摩滅性 耐熱性을 要하는 産業機械, 機關部品用 異種 耐蝕, 耐熱鋼 SUH3-SUH31, SUH3-21-4N의 摩擦壓接과 壓接部의 機械的 特性에 關한 硏究를 遂行한 結果는 다음과 같다.
(1) 各種 材料의 材質과 外徑(??)에 依存하는 摩擦加熱壓 및 加壓力(p, ㎏/㎟)을 摩擦壓接機의 加壓用 油壓실린더 內徑(??) 및 게이지 壓力(??)에 適應시키기 爲해 만든 方程式은 ??이고, 摩擦壓接試驗結果 適用性이 큼이 證明되었다.
(2) 耐熱鋼 SUH3-SUH31 및 SUH3-21-4N의 摩擦壓接을 爲해 決定한 壓接條件 卽, 各各 回轉數 3000rpm, 加熱壓 9.5㎏/㎟, 加壓力 22㎏/㎟, upset量 7㎜ 및 回轉數 3000rpm 加熱壓 8㎏/㎟, 加壓力 20㎏/㎟, upset量 6∼7㎜의 適否를 靜的 試驗과 動的 試驗, 微視的 觀察 및 X-ray探傷 등을 通해 調査한 結果는 溶接缺陷이 全혀 없는 完全한 接合이었으나 壓接部의 硬度分布 不均一緩和, 內部應力 除去, 降伏强度 增加, 衡擊强度 回復, 疲勞壽命 延長과 强度 向上 等을 위해 後熱處理(담금질, 뜨임 溶體化處理 또는 時效處理)가 必要하고, 壓接材 SUH3-SUH31, SUH3-21-4N에 對해 tempering(各各 750。C ×1hr, A.C. 및 600。C ×0.5hr, A.C.)을 施行한 結果는 上述의 效果가 部分的으로 나타났다.
(3) 壓接材의 引張强度는 母材 SUH3과 거의 같았고, 破斷은 母材 SUH3에서 생겼으며, SUH3에 比하여 SUH3-SUH31과 SUH3-21-4N壓接材의 降伏强度는 各各 約 4%, 20%程度 增加하였다.
(4) 回轉數 3000rpm, 加熱壓 8㎏/㎟, 加壓力 20㎏/㎟下에서 SUH3-21-4N 壓接材의 衡擊値를 回復시킬 수 있는 upset量은 6∼7㎜였으며, 이 때 600。C ×300min, A.C.의 tempering 結果 母材 SUH3, 21-4N의 境遇는 各各 約 35%, 48%나 減少한 反面에, 壓接材 SUH3-21-4N의 境遇 衡擊强度의 回復은 約 35.7%였다.
(5) 壓接部의 組織檢査에서 壓接中 生成된 酸化크롬이나 鋼의 製造中 包含된 介在物이 摩擦壓接過程에서 upset量과 더불어 外部로 流出됨이 確認되었고, 一般 耐熱材料의 溶接때의 脫酸齊나 析出 등의 影響으로 생기는 크랙 等의 缺陷이 일어나지 아니 하였다.
(6) 壓接材의 常溫 疲勞强度는 抵下되는 傾向이 있으나, 高溫(700。C) 疲勞强度는 SUH3 母材보다 約 18.2% 增加하였고, 750。C ×1hr, A.C.의 tempering에 의해 末熱處理材보다 高溫 疲勞壽命이 約 16倍나 增加하였다.
(7) 常溫 疲勞試驗에서 摩擦壓接材는 母材보다 壽命係數가 抵下되나, 高溫(700。C)에서는 摩擦壓接材의 壽命係數는 常溫 壓接材에 比해 約 334% 높은 값을 가지며, S-N曲線에서의 折點은 高溫의 境遇가 常溫의 境遇보다 約 2倍의 ??값을 가졌다.
(8) 壓接材의 高溫(700。C) 回轉屈曲 疲勞限度는 單一材인 SUH3에 비하여 約 18.2% 높고, SUH31에 比하여 約 33.3% 낮으며, 壓接材의 常溫 疲勞限度에 比하여 約 63.9% 낮은 값을 가졌다. 그러나 10⁴cycles에서의 時間强度는 末熱處理 平滑材인 壓接材의 境遇가 가장 높고, 熱處理 노치材의 境遇가 가장 낮았다.
(9) 노치感度係數는 SUH31보다 SUH3이 높고, 疲勞限度는 SUH3 노치材가 平滑材보다 約 54.8%, SUH31 노치材는 約 35.3%나 떨어졌다.
(10) 노치 壓接材는 平滑材보다 高溫 疲勞强度가 約 54.6% 낮고, 熱處理 노치 壓接材는 平滑材보다 約 61.5% 減少하나, 末熱處理 노치 壓接材보다는 不過 約 15.3%(0.9㎏/㎟) 減少에 不過하였다.
(11) 回轉屈曲疲勞試驗에서 얻은 實驗値에 對한 S-N曲線의 實驗式은 計算과 實驗에 依해 그 基本型을 PhaseⅡ에 對한 式 ?? 및 PhaseⅡ와Ⅲ을 爲한 式 ??로 定하고, 2次 多項式의 最小自乘法을 電算化하여 digital computer에 依해 計算한 結果式은 實驗値에 거의 一致내지 接近하였으며, 그 信賴度가 各各 約 94.55%, 98.04%였다.
(12) 摩擦壓接材의 回轉屈曲 疲勞試驗에서 高溫疲勞크랙의 發生, 傳播, 成長 및 破斷面의 형상 등은 常溫, 高溫에서 母材의 境遇와 恰似하며, 壓接面에서 보다 母材쪽에서 微小 크랙이 發生, 傳播하고, 引張强度는 거의 같았으나 高溫 疲勞限度가 낮은 SUH3 쪽에서 破斷이 發生하였다.

다국어 초록 (Multilingual Abstract)

Recently, the heat ＆ corrosion resisting alloys have been widely and necessarily used in production of the industrial machine components such as the automobile, aeroplane or marine engines＆ machinery components. Thus, friction welding, the special bonding method for dissimilar steels of such components has been asked for in order to obtain process shortening, production cost reduction, the material characteristics effect of dissimilar metals bonding and so on.
This report deals with the break type friction welding of heat resisting dissimilar materials SUH3-SUH31 and SUH3-21-4N used for engine value materials, etc. and the mechanical properties of welded joints considering on optimum welding condition through static test, microstructural examination, dynamic test(rotary bending fatigue test) at room & elevated temperature and fatigue behavior.
Main results obtained are summarized as follow:
1) The equation for the gage pressure(??) and inside diameter(??) of friction welder's hydraulic piston cylinder versus heating or upsetting pressure(p, ㎏/㎟) depending upon workpiece materials and outside diameter(??) was made and proved useful as follow: ??.
2)Through static and dynamic tests and micro-examination, the optimum welding conditions were rotating speed 3000rpm, heating pressure 9.5㎏/㎟, upsetting pressure 22㎏/㎟, upset amount 7㎜ for SUN3-SUN31, and 3000rpm, 8㎏/㎟, 20㎏/㎟, 6∼7㎜ respectively SUN3-21-4N. However the proper heat treatments after welding such as quenching and tempering, solution treatment or aging treatment are to be carried out to eliminate hardness distribution irregularity and internal stress at welded zone, to increase yielding strength, recover impact strength, lengthen fatigue life and increase fatigue strength.
3) Tensile strength of friction welded joints was almost same as that of parent SUH3 and fracture occurred on parent SUH3. The yielding strength of friction welded joints SUH3-SUH31 and SUH3-21-4N was increased as much as about 4% and 20% respectively.
4) The proper upset amount to recover the impact strength of friction welded joint SUH3-21-4N under rotating speed 3000rpm, heating pressure 8㎏/㎟, upsetting pressure 20㎏/㎟ was 6 through 7㎜. Tempering(600。C ×30min, A.C.) treatment after welding made 35.7% recovery of Charpy’s impact strength in the case of welded joint SUH3-21-4N, while about 35% and 48% decrease respectively in the case of parent SUH3 and 21-4N.
5) By microstructure examination of friction weld interface, it was assured that chrome oxides or incursions were expelled out along with upset during burning off and upsetting process. Thus there were no weld defects such as voids or cracks generally caused by precipitation or deoxidizing agent during welding heat-resisting alloys by other bonding methods.
6) Fatigue strength of friction welded joint SUH3-SUH31 tends to be decreased, but high temperature(700。C) fatigue strength was increased about 18.2% more than parent SUH3 in the rotary bending fatigue test, and the tempering of 750。C ×1hr, A.C. increased the high temperature fatigue life of welded joint SUH3-SUH31 as much as about sixteen times of that of the welded joints as welded.
7) In rotatory bending fatigue test at room temperature, the fatigue life factor of welded joints was decreased more than that of parent metals, but, at high temperature(700。C), it was increased to about 334% comparing with that of room temperature fatigue, and the number of ?? at fatigue limit was increased to about two times.
8) High temperature(700。C) rotary bending fatigue limit of welded joint SUH3-SUH31 was about 18.2% higher than that of base metal SUH3, about 33.3% lower than that of SUH31, about 63.9% lower than room temperature fatigue limit of welded joints. Fatigue strength(at 10⁴cycles) of the welded joints without notch and as welded was the highest, but that of the welded joints with notch and as heat treated was the lowest.
9) Notch sensitivity factor of SUH3 was higher than that of SUH31. So the rotary bending fatigue limit of SUH3 with 1㎜R notch was about 54.8% lower and that of SUH31 with notch was about 35.3% lower than in the case of no notch.
10) High temperature(700。C) fatigue limit of the welded joint SUH3-SUH31 with notch was about 54.6% lower and that of heat treated(750。C ×1hr, A.C.) welded joint SUH3-SUH31 with 1㎜R notch was about 61.5% lower than in the case of no notch, but only about 15.3% (0.9㎏/㎟) lower than in the case of not-heat-treated notched joints.
11) The empirical equations of S-N curves regarding empirical values obtained through rotary bending fatigue test for heat resisting alloys were made by experiments and calculations of digital computer, and the typical equation types are as follows:
??
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The % confidendence of equations was about 94.6%, 98>0% respectively.
12) High temperature fatigue crack initiation in rotary bending fatigue test for friction welded joint SUH3-SUH31, its propagation and growth, fracture characteristics, inclusion effects, etc. were almost similar as in the case of base metals but the hair crack initiation and propagation occurred on parent materials rather than on weld interface, and the fracture occurred on the parent SUH3 with similar tensile strength but low fatigue limit.