鋼材之擠出磨耗機制與理論之探討

Abstract

本文使用Falex摩擦試驗機，以中碳鋼S45C與模具鋼SKD11為材料進行銷盤磨耗試驗。當材料在高滑動速度及足夠高接觸壓力條件下對磨時，銷產生嚴重的磨耗。試驗中藉由觀察接觸面邊緣，發現薄層磨屑從滑動接觸的垂直方向被擠出；此時，另一部份以薄片磨屑型態沿滑動方向被擠出，一些堆積於銷的邊緣，一些磨屑形成火花噴出。此種現象不同於氧化、黏著、刮磨…等磨耗機構產生的結果，定義為擠出磨耗。 本文主要針對鋼的擠出磨耗與其轉換成氧化磨耗之磨耗過程及其顯微組織做分析及探討，再利用相關力學理論推導比較擠出磨耗量。 微觀分析中碳鋼/模具鋼銷結果顯示，在擠出過程中，銷接觸面周圍環繞擠出疊層，當疊層堆疊到一較大體積，其功用如散熱片，造成外觀接觸溫度下降，磨耗率跟著降低。此時，若外觀接觸溫度維持在600℃/700℃以上，銷仍可維持在擠出磨耗機制。若外觀接觸溫度下降至550℃以下，磨耗機制由擠出磨耗轉變成氧化磨耗。 顯微分析結果顯示，在擠出磨耗情況下，銷接觸表面軟化造成塑性層材料連續被擠出，試驗中無明顯氧化物，銷接觸面上很薄的氧化物主要為試驗停止後降溫所產生。而中碳鋼銷接近接觸面之金相組織為細波來鐵，細波來鐵下方存在粗大白色肥粒鐵組織，應該是在滑動過程中由麻田散鐵高溫回火(硬化銷)或再結晶之晶粒成長(軟銷)而成。模具鋼銷接近接觸面之組織，為未回火麻田散鐵基地內散佈顆粒狀碳化物及合金碳化物，而在未回火麻田散鐵下方距接觸面稍遠處，存在高溫回火麻田散鐵基地散佈碳化物組織。 中碳鋼銷經硬化處理後會增加擠出機制初始階段時滑動之摩擦力。而進入穩定擠出磨耗期後，因銷之真實接觸溫度通常大於900℃以上，此時的銷硬度軟化，其初始硬度不會影響穩定擠出磨耗時的硬度。因圓盤溫度較低，更新之接觸點硬度大於銷接觸點硬度很多，所以此時滑動剪力或摩擦力主要由銷的硬度決定。因此，經硬化及未經硬化處理之銷，其外觀接觸溫度以及摩擦係數幾乎相同。 不管銷的磨耗量之實驗值或理論值，其皆隨負載及相對滑動速度增加而增加，符合整個擠出磨耗趨勢。銷磨耗量之實驗值大於理論值，差距在1.7~5.5倍之間，應為其他因素所影響，若經適當修正，便可證實採用塑性力學分析擠出磨耗為一可行方向。

A Falex machine was used to perform the pin-on-disk wear tests. Two kinds of steel which included medium carbon steel and die steel were used. When sliding contact was under sufficiently high pressures and speeds, the pin wear became severe. By observing the edge of the contact interface during a test, we found that thin layers of debris were extruded out from the contact in the direction perpendicular to that of sliding. At the same time, flakes of debris were extruded from the contact in the sliding direction. Some of them piled up at the periphery of the pin, the rest formed sparks. Such phenomena are different from those resulting from oxidation, adhesive and abrasive wear mechanisms. The mechanism of this kind is now termed as extrusion wear. This study investigated extrusion wear mechanisms and transition of wear mechanisms of steel by micro-analyzing the wear surfaces and using mechanics of plasticity to evaluate and compare the real values and theoretical values of pin wear. Micro-analysis of medium carbon steel/ die steel pin shows that many extruded laminated layers were around the rim of the pins in the extrusion process. When these laminated layers accumulated to a bigger volume, they functioned as cooling fins. The contact temperature began to decrease and approached equilibrium. In this scenario, wear rate also decreased. However, the nominal contact temperature was maintained at over 600℃/700℃ in an extrusion wear regime. If the nominal contact temperature decreased to <550 °C, the wear regime changed to an oxidation one, and the extrusion process stopped immediately. In an extrusion wear condition, micro-analysis shows that layers of material were extruded continuously due to softening of the contact surface of the pin. Under such a condition, oxide films of negligible thickness formed during cooling in air after the wear test. The microstructure immediately close to the contact surface of medium carbon steel pin was fine pearlite, which was transformed from austenite at the end of the wear test. Below the fine pearlite zone, a coarse ferrite structure existed that resulted from martensite tempering (for hardened pins) or grain growth at a recrystallization temperature (for as-fabricated pins) during the sliding process. The microstructure close to the contact surface of a die steel pin was spheroidized carbide and main alloy carbide dispersed in untempered martensite base. Additionally, there were carbides dispersed in the tempered martensite base below untempered martensite. The hardening treatment of pins may influence friction and contact temperatures during the initial extrusion wear stage. When the contact reached a stable extrusion wear period, the real contact temperature usually exceeded 900 °C. In such conditions, the initial hardness of the pins did not affect the hardness of the contact surfaces in stable extrusion wear when running conditions were the same. Additionally, as the nominal contact temperature of the disc was lower than that of the pin, the sliding shear force or frictional force was mainly determined with the pin hardness. Therefore, the contact temperatures and friction coefficients for different heat-treated pins were almost the same. The real values or theoretical values of pin wear increase with normal load and sliding speed increasing, such a condition is consistent with the trend of extrusion wear. The results show that the real values of wear are of 1.7 to 5.5 times greater than the theoretical values. This inconsistency should be caused by some facts that were neglected in calculation. If the influent factors are modified appropriately, it will indicate that the plastic analysis is a feasible method to analyze the behavior of extrusion wear.