麻田散鐵型不□鋼帶極潛弧硬面銲覆耐熱疲勞與耐磨耗特性之研究

Abstract

本研究旨在探討合金元素與銲後熱處理對麻田散鐵型不□鋼銲帶潛弧硬面銲覆(Submerged Arc Cladding)耐熱疲勞特性與耐磨耗特性之影響。

實驗中採用PFB-131S與PFB-132兩種不同成分的410不□鋼銲帶為銲覆材料，以SS400結構碳鋼為基材，採用潛弧銲法實施平面堆積銲 (Bead on Plate)，再對兩種銲件施以 625℃、650℃、675℃之銲後回火處理後，以洛氏硬度試驗機量測其熔填金屬硬度，再分別以自製改良的熱循環實驗爐進行耐熱疲勞實驗，量測其疲勞裂縫的裂縫數、總裂縫長度、最大裂縫深度及裂縫長度分布等；復以雙金屬滾輪磨耗試驗機進行20,000轉的滾滑動磨耗試驗，量取其外徑損失量以及重量損失量，以評估不同成分的兩種焊件的耐熱疲勞性及耐磨耗性，以及回火溫度對兩種焊件耐熱疲勞性及耐磨耗性之影響。

而後以光學顯微鏡、掃描式電子顯微鏡，以及穿透式電子顯微鏡等，觀察其直銲下、回火後以及熱疲勞實驗後的顯微組織。再以化學分光儀、X-ray能量色散光譜儀以及擇域繞射，分析母材、銲材與銲件熔填金屬化學成分、熔填金屬中鎳與鉻的分布，以及析出物的結構組成，以瞭解材料成份及銲後回火溫度與熔填金屬的顯微組織、硬度、耐熱疲勞性與耐磨耗性之關係，並推斷其熱疲勞破裂機構。

實驗結果分析得知，隨著回火溫度增加，熔填金屬硬度降低，銲件的耐熱疲勞性與耐磨耗性增加。PFB-131S的硬度、耐磨耗性及耐熱疲勞性皆高於PFB-132。兩種焊件在直銲下的顯微組織皆為麻田散鐵組織，回火後逐漸轉換為連續的肥粒鐵基地，以及不連續的析出物。 410 麻田散鐵不□鋼之碳化物為面心立方結構的(Fe,Cr)23C6型，添加Mo元素，其碳化物變為複雜面心結構的(Fe,Cr,Mo) 23C6型，其析出溫度較低，成顆粒狀，量多且細，並成均勻分布，對硬度、耐磨耗性與耐熱疲勞性都較為有利。添加Ni元素會降低材料AC1溫度，使回火軟化效果明顯，並使碳化物易於晶界析出，且成長條狀，造成晶界弱化，對硬度、耐磨耗性以及耐熱疲性，都有不利的影響。

麻田散鐵型不□鋼的熱疲勞破裂機構，乃是在高溫環境中碳化物的析出，並聚集在晶界與次晶界，於熱疲勞循環應力與殘留應力共同作用下，使基材與析出物介面產生微隙，並隨熱循環應力作用而成長、串聯成微裂縫，最後在熱循環應力與腐蝕雙重作用下，裂縫成長而破裂。

The aim of this study was to investigate the effect of chemical composition and post-weld heat treatment on the thermal fatigue and wear behavior of the submerged arc martensitic stainless steel strip cladding.

Stainless steel strips used for deposited metal were PFB-131S and PFB-132. Cladding on a carbon steel substrate (SS41) was performed using a submerged arc welding process to make bead-on-plate welds. Then the specimens were tempering for post-weld heat treatment at 625, 650 and 675 degree Celsius. The deposited metals were measured using the Rockwell C scale. A newly designed thermal fatigue testing apparatus was used to evaluate total crack length, maximum crack length and crack distribution.

An Amsler type wear testing machine was used to investigate the wear behaviour. The weight and dimension loss were measured after 20,000 revolutions. In addition, the thermal fatigue and wear resistance were evaluated in different strips and tempering temperature.

In order to understand the effects of chemical composition and tempering temperature on the microstructure, hardness, thermal fatigue, wear resistance and failure mechanism. The microstructure of the deposited metal such as as-welded, tempering treatment and thermal fatigue test were analyzed by means of optical microscopy, scanning electron microscopy and transmission electron microscopy. The spectrometer, X-ray energy dispersion spectroscopy and selected area diffraction pattern were used to examine the chemical composition of substrate, strips and deposited metals. In addition, the quantity of chrome and nickel in the deposited metal, crystal structure and morphology of the precipitates were also evaluated.

The results showed that increasing the tempering temperature not only decreased the hardness but also improved thermal fatigue and wear resistance. Besides, the PFB-131S specimens were significantly higher than that of the PFB-132 specimens. Deposited metal on both specimens were martensitic structure. When tempering treatment was finished, the microstructure with columnar grains can be decomposed to form alpha-ferrite and the precipitates became fine particles.

The precipitates of martensitic stainless steel had a chromium-rich FCC (Fe,Cr)23C6 structure. The FCC (Fe,Cr,Mo) 23C6 complex carbides were associated with high molybdenum content in the deposited metal. Furthermore, the precipitation temperatures of molybdenum carbides were lower than that of chromium carbides. The fine carbide particles relatively evenly distributed in the matrix, which exhibited a favorable hardness, thermal fatigue and wear resistance. Nickel can reduce the AC1 temperature, which tends to have a significant tendency toward temper softening. Carbides can easily develop along the grain boundaries as AC1 temperature decreased. As a result, grain boundaries became weaker and had a negative effect on hardness, thermal fatigue and wear resistance.

The mechanism of the thermal fatigue failure was the precipitates along grain and sub-grain boundaries were formed at high temperature. Due to the thermal cycles stress and the residual stress, micro fissures or cavities appear near the precipitate-matrix interface, the microvoid grows with increasing thermal cycles. Because of the microvoid linking occurred, caused grain boundaries cracks under combined the action of thermal stress and corrosion.